descriptors does not fit into currently allocated LDT, or trim the
return if the range fits partially. Before, the function returned
EINVAL.
Fix two bugs in r324366: use capped num counter for malloc size, and
do not leak allocated buffer on EINVAL (by handling EINVAL case as
normal, see above).
Reviewed by: bde
Sponsored by: The FreeBSD Foundation
MFC after: 1 week