devpts: hoist out check for DEVPTS_SUPER_MAGIC

Hoist the check whether we have already found a suitable devpts filesystem
out of devpts_ptmx_path() in preparation for the devpts bind-mount
resolution patch. This is a non-functional change.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>