udmabuf: udmabuf_create pin folio codestyle cleanup

This patch aim to simplify the memfd folio pin during the udmabuf
create. No functional changes.

This patch create a udmabuf_pin_folios function, in this, do the memfd
pin folio and then record each pinned folio, offset.

This patch simplify the pinned folio record, iter by each pinned folio,
and then record each offset in it.

Compare to iter by pgcnt, more readable.

Suggested-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Signed-off-by: Huan Yang <link@vivo.com>
Acked-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240918025238.2957823-5-link@vivo.com