Similar to the addition of lookup_one() add a version of
lookup_one_unlocked() and lookup_one_positive_unlocked() that take
idmapped mounts into account. This is required to port overlay to
support idmapped base layers.

Cc: <linux-fsdevel@vger.kernel.org>
Tested-by: NGiuseppe Scrivano <gscrivan@redhat.com>
Reviewed-by: NAmir Goldstein <amir73il@gmail.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NChristian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: NMiklos Szeredi <mszeredi@redhat.com>