simplify the callers of mnt_unhold_writers()

simplify the callers of mnt_unhold_writers()

The logics in cleanup on failure in mount_setattr_prepare() is simplified
by having the mnt_hold_writers() failure followed by advancing m to the
next node in the tree before leaving the loop.

And since all calls are preceded by the same check that flag has been set
and the function is inlined, let's just shift the check into it.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>