fs: Fix missed inode writeback when racing with __writeback_single_inode

fs: Fix missed inode writeback when racing with __writeback_single_inode

When mmb_fsync_noflush() or simple_fsync_noflush() race with another
writeback of the same inode, they can see inode dirty bits are already
clear and skip inode writeback although the racing
__writeback_single_inode() didn't yet get to writing anything. This can
result in fsync(2) returning without properly persisting the inode.

We already have I_SYNC bit for this synchronization and
writeback_single_inode() properly uses it so just fix
mmb_fsync_noflush() and simple_fsync_noflush() to take it into account
as well.

Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20260727104923.3828017-23-jack@suse.cz
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>