libceph: don't clear bio_iter in prepare_write_message()

At one time it was necessary to clear a message's bio_iter field to
avoid a bad pointer dereference in write_partial_msg_pages().

That no longer seems to be the case.  Here's why.

The message's bio fields represent (in this case) outgoing data.
Between where the bio_iter is made NULL in prepare_write_message()
and the call in that function to prepare_message_data(), the
bio fields are never used.

In prepare_message_data(), init-bio_iter() is called, and the result
of that overwrites the value in the message's bio_iter field.

Because it gets overwritten anyway, there is no need to set it to
NULL.  So don't do it.

Signed-off-by: Alex Elder <elder@inktank.com>
libceph: use local variables for message positions

There are several places where a message's out_msg_pos or in_msg_pos
field is used repeatedly within a function.  Use a local pointer
variable for this purpose to unclutter the code.

Signed-off-by: Alex Elder <elder@inktank.com>
libceph: consolidate message prep code

In prepare_write_message_data(), various fields are initialized in
preparation for writing message data out.  Meanwhile, in
read_partial_message(), there is essentially the same block of code,
operating on message variables associated with an incoming message.

Generalize prepare_write_message_data() so it works for both
incoming and outcoming messages, and use it in both spots.  The
did_page_crc is not used for input (so it's harmless to initialize
it).

Signed-off-by: Alex Elder <elder@inktank.com>
libceph: small write_partial_msg_pages() refactor

Define local variables page_offset and length to represent the range
of bytes within a page that will be sent by ceph_tcp_sendpage() in
write_partial_msg_pages().

Signed-off-by: Alex Elder <elder@inktank.com>
libceph: encapsulate reading message data

Pull the code that reads the data portion into a message into
a separate function read_partial_msg_data().

Rename write_partial_msg_pages() to be write_partial_message_data()
to match its read counterpart, and to reflect its more generic
purpose.

Signed-off-by: Alex Elder <elder@inktank.com>
libceph: define and use ceph_tcp_recvpage()

Define a new function ceph_tcp_recvpage() that behaves in a way
comparable to ceph_tcp_sendpage().

Rearrange the code in both read_partial_message_pages() and
read_partial_message_bio() so they have matching structure,
(similar to what's in write_partial_msg_pages()), and use
this new function.

Signed-off-by: Alex Elder <elder@inktank.com>
libceph: define and use ceph_crc32c_page()

Factor out a common block of code that updates a CRC calculation
over a range of data in a page.

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