Bring truncate.c's code for truncate_inode_pages_range() inline into

Bring truncate.c's code for truncate_inode_pages_range() inline into
shmem_truncate_range(), replacing its first call (there's a followup call
below, but leave that one, it will disappear next).

Don't play with it yet, apart from leaving out the cleancache flush, and
(importantly) the nrpages == 0 skip, and moving shmem_setattr()'s partial
page preparation into its partial page handling.

Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>