LU-12275 sec: encryption for write path

LU-12275 sec: encryption for write path

First aspect is to make sure encryption context is properly set on
files/dirs that are created or opened/looked up.
Then encryption itself is carried out in osc_brw_prep_request(), just
before pages are added to the request to be sent. Because pages in
the page cache must hold clear text data, we have to use bounce pages
for encryption. The allocation is handled by fscrypt, and for
deallocation we call fscrypt_pullback_bio_page() and/or
fscrypt_pullback_bio_page().

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Ieb1355bd55b6a8740e4b549d60d1f480a5abc53f
Reviewed-on: https://review.whamcloud.com/36144
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>