MFC r280957

MFC r280957

  Fix integer truncation bug in malloc(9)

  A couple of internal functions used by malloc(9) and uma truncated
  a size_t down to an int.  This could cause any number of issues
  (e.g. indefinite sleeps, memory corruption) if any kernel
  subsystem tried to allocate 2GB or more through malloc.  zfs would
  attempt such an allocation when run on a system with 2TB or more
  of RAM.

git-svn-id: svn://svn.freebsd.org/base/stable/10@287945 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f