ext4: add inline encryption support

Wire up ext4 to support inline encryption via the helper functions which
fs/crypto/ now provides.  This includes:

- Adding a mount option 'inlinecrypt' which enables inline encryption
  on encrypted files where it can be used.

- Setting the bio_crypt_ctx on bios that will be submitted to an
  inline-encrypted file.

  Note: submit_bh_wbc() in fs/buffer.c also needed to be patched for
  this part, since ext4 sometimes uses ll_rw_block() on file data.

- Not adding logically discontiguous data to bios that will be submitted
  to an inline-encrypted file.

- Not doing filesystem-layer crypto on inline-encrypted files.

Co-developed-by: Satya Tangirala <satyat@google.com>
Signed-off-by: Satya Tangirala <satyat@google.com>
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Link: https://lore.kernel.org/r/20200702015607.1215430-5-satyat@google.com
Signed-off-by: Eric Biggers <ebiggers@google.com>

Documentation/admin-guide/ext4.rst

@@ -395,6 +395,13 @@ When mounting an ext4 filesystem, the following option are accepted:
         Documentation/filesystems/dax.txt.  Note that this option is
         incompatible with data=journal.
 
+  inlinecrypt
+        When possible, encrypt/decrypt the contents of encrypted files using the
+        blk-crypto framework rather than filesystem-layer encryption. This
+        allows the use of inline encryption hardware. The on-disk format is
+        unaffected. For more details, see
+        Documentation/block/inline-encryption.rst.
+
 Data Mode
 =========
 There are 3 different data modes: