fuse: re-work fuse_get_user_pages() to operate on iovec[]

Now fuse_get_user_pages() takes iovec[] as argument and packs so much data
from it to req->pages[] as possible.

Change-Id: I985d1abbcc9873a786817df0b015c8e733540df4
Signed-off-by: Maxim Patlasov <mpatlasov@parallels.com>
fuse: re-work fuse_direct_io() to operate on iovec[]

The patch transparently passes iovec[] argument from fuse_direct_io()
to fuse_get_user_pages() hoping that the latter would fill req with
some part of iovec[].

Change-Id: I4bb3504af62efecd9cd15d7e0a86cbd6b8fa3638
Signed-off-by: Maxim Patlasov <mpatlasov@parallels.com>