rbd: have __rbd_add_snap_dev() return a pointer

It's not obvious whether the snapshot pointer whose address is
provided to __rbd_add_snap_dev() will be assigned by that function.
Change it to return the snapshot, or a pointer-coded errno in the
event of a failure.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com> 
rbd: make rbd_create_rw_ops() return a pointer

Either rbd_create_rw_ops() will succeed, or it will fail because a
memory allocation failed.  Have it just return a valid pointer or
null rather than stuffing a pointer into a provided address and
returning an errno.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com> 
rbd: pass null version pointer in add_snap()

rbd_header_add_snap() passes the address of a version variable to
rbd_req_sync_exec(), but it ignores the result.  Just pass a null
pointer instead.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com> 
rbd: always pass ops array to rbd_req_sync_op()

All of the callers of rbd_req_sync_op() except one pass a non-null
"ops" pointer.  The only one that does not is rbd_req_sync_read(),
which passes CEPH_OSD_OP_READ as its "opcode" and, CEPH_OSD_FLAG_READ
for "flags".

By allocating the ops array in rbd_req_sync_read() and moving the
special case code for the null ops pointer into it, it becomes
clear that much of that code is not even necessary.

In addition, the "opcode" argument to rbd_req_sync_op() is never
actually used, so get rid of that.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com> 
rbd: fixes in rbd_header_from_disk()

This fixes a few issues in rbd_header_from_disk():
    - There is a check intended to catch overflow, but it's wrong in
      two ways.
	- First, the type we don't want to overflow is size_t, not
	  unsigned int, and there is now a SIZE_MAX we can use for
	  use with that type.
	- Second, we're allocating the snapshot ids and snapshot
	  image sizes separately (each has type u64; on disk they
          grouped together as a rbd_image_header_ondisk structure).
	  So we can use the size of u64 in this overflow check.
    - If there are no snapshots, then there should be no snapshot
      names.  Enforce this, and issue a warning if we encounter a
      header with no snapshots but a non-zero snap_names_len.
    - When saving the snapshot names into the header, be more direct
      in defining the offset in the on-disk structure from which
      they're being copied by using "snap_count" rather than "i"
      in the array index.
    - If an error occurs, the "snapc" and "snap_names" fields are
      freed at the end of the function.  Make those fields be null
      pointers after they're freed, to be explicit that they are
      no longer valid.
    - Finally, move the definition of the local variable "i" to the
      innermost scope in which it's needed.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com> 
rbd: return obj version in __rbd_refresh_header()

Add a new parameter to __rbd_refresh_header() through which the
version of the header object is passed back to the caller.  In most
cases this isn't needed.  The main motivation is to normalize
(almost) all calls to __rbd_refresh_header() so they are all
wrapped immediately by mutex_lock()/mutex_unlock().

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com> 
rbd: create rbd_refresh_helper()

Create a simple helper that handles the common case of calling
__rbd_refresh_header() while holding the ctl_mutex.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>