vfs_mount.c: Allow mountd(8) to do exports in a vnet prison

To run mountd in a vnet prison, three checks in vfs_domount()
and vfs_domount_update() related to doing exports needed
to be changed, so that a file system visible within the
prison but mounted outside the prison can be exported.

I did all three in a minimal way, only changing the checks for
the specific case of a process (typically mountd) doing exports
within a vnet prison and not updating the mount point in other
ways. The changes are:

• Ignore the error return from vfs_suser(), since the file system being mounted outside the prison will cause it to fail.
• Use the priv_check(PRIV_NFS_DAEMON) for this specific case within a prison.
• Skip the call to VFS_MOUNT(), since it will return an error, due to the "from" argument not being set correctly. VFS_MOUNT() does not appear to do anything for the case of doing exports only.

Reviewed by: markj
MFC after: 3 months
Differential Revision: https://reviews.freebsd.org/D37741