Instead of add allocated file_region one by one to region_cache, we could
use list_splice to merge two list at once.

Also we know the number of entries in the list, increase the number
directly.
Signed-off-by: Wei Yang <richard.weiyang@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Baoquan He <bhe@redhat.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Link: https://lkml.kernel.org/r/20200831022351.20916-4-richard.weiyang@linux.alibaba.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>