libceph: make recv path in secure mode work the same as send path

libceph: make recv path in secure mode work the same as send path

The recv path of secure mode is intertwined with that of crc mode.
While it's slightly more efficient that way (the ciphertext is read
into the destination buffer and decrypted in place, thus avoiding
two potentially heavy memory allocations for the bounce buffer and
the corresponding sg array), it isn't really amenable to changes.
Sacrifice that edge and align with the send path which always uses
a full-sized bounce buffer (currently there is no other way -- if
the kernel crypto API ever grows support for streaming (piecewise)
en/decryption for GCM [1], we would be able to easily take advantage
of that on both sides).

[1] https://lore.kernel.org/all/20141225202830.GA18794@gondor.apana.org.au/

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>