mount.nfs: No need to return nfs_mount_data structs

Refactor mount processing slightly to remove an output parameter and an
unnecessary type cast.  The mount syscall is now made from inside
nfs_mount or nfs4mount, rather than in common code after those are called.

Code review suggests that EX_BG was never returned by mount.nfs because the
logic I just replaced was always returning EX_FAIL.  The new logic should
properly return EX_BG when appropriate.

However, it is unclear whether /bin/mount handles backgrounding the mount
request, or whether mount.nfs should.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Neil Brown <neilb@suse.de>