fuse: add basic support of iovec[] to fuse_req

The patch allows fuse_req to refer to array of iovec-s describing
layout of user-data over req->pages. fuse_copy_pages() is re-worked to
support both cased: former layout where pages[] corresponded to <buf, len>
and newer one where pages[] corresponds to iovec[].

Change-Id: I00abeede3d1dc7e9208f18a73acb58c5f2872a81
Signed-off-by: Maxim Patlasov <mpatlasov@parallels.com>