[PATCH] Fix missing ret assignment in __bio_map_user() error path
If get_user_pages() returns less pages than what we asked for, we jump
to out_unmap which will return ERR_PTR(ret). But ret can contain a
positive number just smaller than local_nr_pages, so be sure to set it
to -EFAULT always.
Problem found and diagnosed by Damien Le Moal <damien@sdl.hitachi.co.jp>