fs: fix a struct path leak in path_umount

Make sure we also put the dentry and vfsmnt in the illegal flags
and !may_umount cases.

Fixes: 41525f56 ("fs: refactor ksys_umount")
Reported-by: Vikas Kumar <vikas.kumar2@arm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>