MFC r288281

MFC r288281
  The conversion of kmem_alloc_attr() from operating on a vm map to a vmem
  arena in r254025 introduced a bug in the case when an allocation is only
  partially successful.  Specifically, the vm object lock was not being
  acquired before freeing the allocated pages.  To address this bug, replace
  the existing code by a call to kmem_unback().

  Change the type of a variable in kmem_alloc_attr() so that an allocation
  of two or more gigabytes won't fail.

  Replace the error handling code in kmem_back() by a call to kmem_unback().

git-svn-id: svn://svn.freebsd.org/base/stable/10@288556 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f