capabilities: simplify bound checks for copy_from_user()

The capabilities syscall has a copy_from_user() call where gcc currently
cannot prove to itself that the copy is always within bounds.

This patch adds a very explicity bound check to prove to gcc that this
copy_from_user cannot overflow its destination buffer.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Acked-by: James Morris <jmorris@namei.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: James Morris <jmorris@namei.org>