nsfs: Simplify __ns_get_path

Move mntget from the very beginning of __ns_get_path to
the success path of __ns_get_path, and remove the mntget
calls.

This removes the possibility that there will be a mntget/mntput
pair of __ns_get_path has to retry, and generally simplifies the code.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>