LU-12353 ldiskfs: speedup quota journalling

ldiskfs_mark_dquot_dirty() need not call
ldiskfs_write_dquot() if the dquot is already
dirty since we are guaranteed that another
thread is already writing the dquot and
will update the buffer with the latest
dquot data.

Change-Id: Ia65c5987bbb0d6d2ac94c5499cb8b51edc3c49c3
Signed-off-by: Andrew Perepechko <c17827@cray.com>
Cray-bug-id: LUS-5342
Reviewed-on: https://review.whamcloud.com/34992
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>