ext4, jbd2: add an optimized bmap for the journal inode

[ Upstream commit 62913ae9 ]

The generic bmap() function exported by the VFS takes locks and does
checks that are not necessary for the journal inode.  So allow the
file system to set a journal-optimized bmap function in
journal->j_bmap.

Reported-by:  <syzbot+9543479984ae9e576000@syzkaller.appspotmail.com>
Link: https://syzkaller.appspot.com/bug?id=e4aaa78795e490421c79f76ec3679006c8ff4cf0


Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Sasha Levin <sashal@kernel.org>