mm/hugetlb: add region_del() to delete a specific range of entries

mm/hugetlb: add region_del() to delete a specific range of entries

fallocate hole punch will want to remove a specific range of pages.  The
existing region_truncate() routine deletes all region/reserve map
entries after a specified offset.  region_del() will provide this same
functionality if the end of region is specified as LONG_MAX.  Hence,
region_del() can replace region_truncate().

Unlike region_truncate(), region_del() can return an error in the rare
case where it can not allocate memory for a region descriptor.  This
ONLY happens in the case where an existing region must be split.
Current callers passing LONG_MAX as end of range will never experience
this error and do not need to deal with error handling.  Future callers
of region_del() (such as fallocate hole punch) will need to handle this
error.

Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Aneesh Kumar <aneesh.kumar@linux.vnet.ibm.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>