vfs: fsmount: add missing mntget()

sys_fsmount() needs to take a reference to the new mount when adding it
to the anonymous mount namespace.  Otherwise the filesystem can be
unmounted while it's still in use, as found by syzkaller.

Reported-by: Mark Rutland <mark.rutland@arm.com>
Reported-by:  <syzbot+99de05d099a170867f22@syzkaller.appspotmail.com>
Reported-by:  <syzbot+7008b8b8ba7df475fdc8@syzkaller.appspotmail.com>
Fixes: 93766fbd

 ("vfs: syscall: Add fsmount() to create a mount for a superblock")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>