bio_map_kern() returns an ERR_PTR() not NULL.

Found by smatch (http://repo.or.cz/w/smatch.git).  Compile tested.
Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>