Add a mount option to support JBD2 feature:

JBD2_FEATURE_INCOMPAT_ASYNC_COMMIT.  When this feature is opened, journal
commit block can be written to disk without waiting for descriptor blocks,
which can improve journal commit performance.  This option will enable
'journal_checksum' internally.

Using the fs_mark benchmark, using journal_async_commit shows a 50%
improvement, the files per second go up from 215.2 to 317.5.

test script:
fs_mark  -d  /mnt/ocfs2/  -s  10240  -n  1000

default:
FSUse%        Count         Size    Files/sec     App Overhead
     0         1000        10240        215.2            17878

with journal_async_commit option:
FSUse%        Count         Size    Files/sec     App Overhead
     0         1000        10240        317.5            17881
Signed-off-by: Alex Chen <alex.chen@huawei.com>
Signed-off-by: Weiwei Wang <wangww631@huawei.comm>
Reviewed-by: Joseph Qi <joseph.qi@huawei.com>
Reviewed-by: Mark Fasheh <mfasheh@suse.de>
Cc: Joel Becker <jlbec@evilplan.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>