bio_alloc() with __GFP_WAIT never returns NULL

Mark the error handling branch as unlikely() because the current
kernel interface can never return NULL. However, we want to keep
the error handling in case this behavior changes in the futre.

Plus fix a small style issue.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Isaac Huang <he.huang@intel.com>
Closes #2703