fat: only specify I_DIRTY_TIME when needed in fat_update_time()

fat: only specify I_DIRTY_TIME when needed in fat_update_time()

As was done for generic_update_time(), only pass I_DIRTY_TIME to
__mark_inode_dirty() when the inode's timestamps were actually updated
and lazytime is enabled.  This avoids a weird edge case where
I_DIRTY_TIME could be set in i_state when lazytime isn't enabled.

Link: https://lore.kernel.org/r/20210112190253.64307-5-ebiggers@kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jan Kara <jack@suse.cz>