Avoid calling mnt_user_ns() many time in
a function.

Cc: Christoph Hellwig <hch@infradead.org>
Cc: Christian Brauner <christian@brauner.io>
Signed-off-by: NHyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: NNamjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: NSteve French <stfrench@microsoft.com>