libceph: fix possible CONFIG_BLOCK build problem

This patch:
    15a0d7b libceph: record message data length
did not enclose some bio-specific code inside CONFIG_BLOCK as
it should have.  Fix that.

Signed-off-by: Alex Elder <elder@inktank.com>
libceph: skip message if too big to receive

We know the length of our message buffers.  If we get a message
that's too long, just dump it and ignore it.

This resolves:
    http://tracker.ceph.com/issues/4664

Signed-off-by: Alex Elder <elder@inktank.com>
libceph: record bio length

The bio is the only data item type that doesn't record its full
length.  Fix that.

Signed-off-by: Alex Elder <elder@inktank.com>
libceph: move cursor into message

A message will only be processing a single data item at a time, so
there's no need for each data item to have its own cursor.

Move the cursor embedded in the message data structure into the
message itself.  To minimize the impact, keep the data->cursor
field, but make it be a pointer to the cursor in the message.

Move the definition of ceph_msg_data above ceph_msg_data_cursor so
the cursor can point to the data without a forward definition rather
than vice-versa.

This and the upcoming patches are part of:
    http://tracker.ceph.com/issues/3761

Signed-off-by: Alex Elder <elder@inktank.com>
libceph: have cursor point to data

Rather than having a ceph message data item point to the cursor it's
associated with, have the cursor point to a data item.  This will
allow a message cursor to be used for more than one data item.

Signed-off-by: Alex Elder <elder@inktank.com>
libceph: replace message data pointer with list

In place of the message data pointer, use a list head which links
through message data items.  For now we only support a single entry
on that list.

Signed-off-by: Alex Elder <elder@inktank.com>
libceph: implement multiple data items in a message

This patch adds support to the messenger for more than one data item
in its data list.

A message data cursor has two more fields to support this:
    - a count of the number of bytes left to be consumed across
      all data items in the list, "total_resid"
    - a pointer to the head of the list (for validation only)

The cursor initialization routine has been split into two parts: the
outer one, which initializes the cursor for traversing the entire
list of data items; and the inner one, which initializes the cursor
to start processing a single data item.

When a message cursor is first initialized, the outer initialization
routine sets total_resid to the length provided.  The data pointer
is initialized to the first data item on the list.  From there, the
inner initialization routine finishes by setting up to process the
data item the cursor points to.

Advancing the cursor consumes bytes in total_resid.  If the resid
field reaches zero, it means the current data item is fully
consumed.  If total_resid indicates there is more data, the cursor
is advanced to point to the next data item, and then the inner
initialization routine prepares for using that.  (A check is made at
this point to make sure we don't wrap around the front of the list.)

The type-specific init routines are modified so they can be given a
length that's larger than what the data item can support.  The resid
field is initialized to the smaller of the provided length and the
length of the entire data item.

When total_resid reaches zero, we're done.

This resolves:
    http://tracker.ceph.com/issues/3761

Signed-off-by: Alex Elder <elder@inktank.com>
libceph: add, don't set data for a message

Change the names of the functions that put data on a pagelist to
reflect that we're adding to whatever's already there rather than
just setting it to the one thing.  Currently only one data item is
ever added to a message, but that's about to change.

This resolves:
    http://tracker.ceph.com/issues/2770

Signed-off-by: Alex Elder <elder@inktank.com>
libceph: make method call data be a separate data item

Right now the data for a method call is specified via a pointer and
length, and it's copied--along with the class and method name--into
a pagelist data item to be sent to the osd.  Instead, encode the
data in a data item separate from the class and method names.

This will allow large amounts of data to be supplied to methods
without copying.  Only rbd uses the class functionality right now,
and when it really needs this it will probably need to use a page
array rather than a page list.  But this simple implementation
demonstrates the functionality on the osd client, and that's enough
for now.

This resolves:
    http://tracker.ceph.com/issues/4104

Signed-off-by: Alex Elder <elder@inktank.com>