ext4: unify the ext4_handle_release_buffer() api

There are two wrapper functions which do exactly the same thing:
ext4_journal_release_buffer(), and ext4_handle_release_buffer().  In
addition, ext4_xattr_block_set() calls jbd2_journal_release_buffer()
directly.

Unify all of the code to use ext4_handle_release_buffer(), and get rid
of ext4_journal_release_buffer().

Signed-off-by: Amir Goldstein <amir73il@users.sf.net>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>