jbd2: fix race between write_metadata_buffer and get_write_access

The function jbd2_journal_write_metadata_buffer() calls
jbd_unlock_bh_state(bh_in) too early; this could potentially allow
another thread to call get_write_access on the buffer head, modify the
data, and dirty it, and allowing the wrong data to be written into the
journal.  Fortunately, if we lose this race, the only time this will
actually cause filesystem corruption is if there is a system crash or
other unclean shutdown of the system before the next commit can take
place.

Signed-off-by: dingdinghua <dingdinghua85@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>