mm/hugetlb: add cache of descriptors to resv_map for region_add

hugetlbfs is used today by applications that want a high degree of
control over huge page usage.  Often, large hugetlbfs files are used to
map a large number huge pages into the application processes.  The
applications know when page ranges within these large files will no
longer be used, and ideally would like to release them back to the
subpool or global pools for other uses.  The fallocate() system call
provides an interface for preallocation and hole punching within files.
This patch set adds fallocate functionality to hugetlbfs.

fallocate hole punch will want to remove a specific range of pages.
When pages are removed, their associated entries in the region/reserve
map will also be removed.  This will break an assumption in the
region_chg/region_add calling sequence.  If a new region descriptor must
be allocated, it is done as part of the region_chg processing.  In this
way, region_add can not fail because it does not need to attempt an
allocation.

To prepare for fallocate hole punch, create a "cache" of descriptors
that can be used by region_add if necessary.  region_chg will ensure
there are sufficient entries in the cache.  It will be necessary to
track the number of in progress add operations to know a sufficient
number of descriptors reside in the cache.  A new routine region_abort
is added to adjust this in progress count when add operations are
aborted.  vma_abort_reservation is also added for callers creating
reservations with vma_needs_reservation/vma_commit_reservation.

[akpm@linux-foundation.org: fix typo in comment, use more cols]
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>