xfs: Fix spelling mistake "drity" -> "dirty"

There is a spelling mistake in fs/xfs/xfs_log.c. Fix it.

Signed-off-by: Zhang Xianwei <zhang.xianwei8@zte.com.cn>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>