more bio_map_user_iov() leak fixes

commit 2b04e8f6bbb196cab4b232af0f8d48ff2c7a8058 upstream.

we need to take care of failure exit as well - pages already
in bio should be dropped by analogue of bio_unmap_pages(),
since their refcounts had been bumped only once per reference
in bio.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
[bwh: Backported to 3.2: adjust filename, context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --cc fs/bio.c
Simple merge