jbd2: speedup jbd2_journal_dirty_metadata()

jbd2: speedup jbd2_journal_dirty_metadata()

It is often the case that we mark buffer as having dirty metadata when
the buffer is already in that state (frequent for bitmaps, inode table
blocks, superblock). Thus it is unnecessary to contend on grabbing
journal head reference and bh_state lock. Avoid that by checking whether
any modification to the buffer is needed before grabbing any locks or
references.

[ Note: this is a fixed version of commit 3edbf70611553, which was
  reverted in 6e22565645ebf51 due to a false positive triggering of an
  assertion check. -- Ted ]

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>