hugetlb: remove duplicated code

Remove duplicated code between region_chg and region_add, and refactor
it into a common function, add_reservation_in_range.  This is mostly
done because there is a follow up change in another series that disables
region coalescing in region_add, and I want to make that change in one
place only.  It should improve maintainability anyway on its own.

[akpm@linux-foundation.org: coding style fixes]
Link: http://lkml.kernel.org/r/20190919200428.188797-3-almasrymina@google.com


Signed-off-by: Mina Almasry <almasrymina@google.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Greg Thelen <gthelen@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>