LU-15003 sec: use enc pool for bounce pages

LU-15003 sec: use enc pool for bounce pages

Take pages from the enc pool so that they can be used for
encryption, instead of letting llcrypt allocate a bounce page
for every call to the encryption primitives.
Pages are taken from the enc pool a whole array at a time.

This requires modifying the llcrypt API, so that new functions
llcrypt_encrypt_page() and llcrypt_decrypt_page() are exported.
These functions take a destination page parameter.
Until this change is pushed in upstream fscrypt, this performance
optimization is not available when Lustre is built and run against
the in-kernel fscrypt lib.

Using enc pool for bounce pages is a worthwhile performance win. Here
are performance penalties incurred by encryption, without this patch,
and with this patch:

                     ||=====================|=====================||
                     || Performance penalty | Performance penalty ||
                     ||    without patch    |     with patch      ||
||==========================================|=====================||
|| Bandwidth – write |        30%-35%       |   5%-10% large IOs  ||
||                   |                      |    15% small IOs    ||
||------------------------------------------|---------------------||
|| Bandwidth – read  |         20%          |    less than 10%    ||
||------------------------------------------|---------------------||
||      Metadata     |         N/A          |         5%          ||
|| creat,stat,remove |                      |                     ||
||==========================================|=====================||

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Change-Id: I3078d0a3349b3d24acc5e61ab53ac434b5f9d0e3