orangefs_kill_sb(): deal with allocation failures

commit 659038428cb43a66e3eff71e2c845c9de3611a98 upstream.

orangefs_fill_sb() might've failed to allocate ORANGEFS_SB(s); don't
oops in that case.

Cc: stable@kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>