fix unbalanced page refcounting in bio_map_user_iov

fix unbalanced page refcounting in bio_map_user_iov

commit 95d78c28b5a85bacbc29b8dba7c04babb9b0d467 upstream.

bio_map_user_iov and bio_unmap_user do unbalanced pages refcounting if
IO vector has small consecutive buffers belonging to the same page.
bio_add_pc_page merges them into one, but the page reference is never
dropped.

Signed-off-by: Vitaly Mayatskikh <v.mayatskih@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
[bwh: Backported to 3.2: adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>