bio_map_kern() returns ERR_PTRs on failure and never returns NULL.

[jejb: remove redundant unlikely spotted by Tobias Klauser]
Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Boaz Harrosh <bharrosh@panasas.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>