fscrypt: allow synchronous bio decryption

Cherry-pick from origin/upstream-f2fs-stable-linux-4.9.y:
  0509923b ("fscrypt: allow synchronous bio decryption")

Currently, fscrypt provides fscrypt_decrypt_bio_pages() which decrypts a
bio's pages asynchronously, then unlocks them afterwards.  But, this
assumes that decryption is the last "postprocessing step" for the bio,
so it's incompatible with additional postprocessing steps such as
authenticity verification after decryption.

Therefore, rename the existing fscrypt_decrypt_bio_pages() to
fscrypt_enqueue_decrypt_bio().  Then, add fscrypt_decrypt_bio() which
decrypts the pages in the bio synchronously without unlocking the pages,
nor setting them Uptodate; and add fscrypt_enqueue_decrypt_work(), which
enqueues work on the fscrypt_read_workqueue.  The new functions will be
used by filesystems that support both fscrypt and fs-verity.

Change-Id: I3e39e4f2c38726664b01537b6c53fae674d7a3ee
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>