btrfs: add helper folio_end()

btrfs: add helper folio_end()

There are several cases of folio_pos + folio_size, add a convenience
helper for that. This is a local helper and not proposed as folio API
because it does not seem to be heavily used elsewhere:

A quick grep (folio_size + folio_end) in fs/ shows

     24 btrfs
      4 iomap
      4 ext4
      2 xfs
      2 netfs
      1 gfs2
      1 f2fs
      1 bcachefs
      1 buffer.c

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>