NFS: NFSv2/NFSv3: Use cred from fs_context during mount

There was refactoring done to use the fs_context for mounting done in:
62a55d08: NFS: Additional refactoring for fs_context conversion

This made it so that the net_ns is fetched from the fs_context (the netns
that fsopen is called in). This change also makes it so that the credential
fetched during fsopen is used as well as the net_ns.

NFS has already had a number of changes to prepare it for user namespaces:
1a58e8a0: NFS: Store the credential of the mount process in the nfs_server
264d948c: NFS: Convert NFSv3 to use the container user namespace
c207db2f: NFS: Convert NFSv2 to use the container user namespace

Previously, different credentials could be used for creation of the
fs_context versus creation of the nfs_server, as FSCONFIG_CMD_CREATE did
the actual credential check, and that's where current_creds() were fetched.
This meant that the user namespace which fsopen was called in could be a
non-init user namespace. This still requires that the user that calls
FSCONFIG_CMD_CREATE has CAP_SYS_ADMIN in the init user ns.

This roughly allows a privileged user to mount on behalf of an unprivileged
usernamespace, by forking off and calling fsopen in the unprivileged user
namespace. It can then pass back that fsfd to the privileged process which
can configure the NFS mount, and then it can call FSCONFIG_CMD_CREATE
before switching back into the mount namespace of the container, and finish
up the mounting process and call fsmount and move_mount.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Tested-by: Alban Crequy <alban.crequy@gmail.com>
Fixes: 62a55d08 ("NFS: Additional refactoring for fs_context conversion")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>