afs: Wrap most op->error accesses with inline funcs

Wrap most op->error accesses with inline funcs which will make it easier
for a subsequent patch to replace op->error with something else.  Two
functions are added to this end:

 (1) afs_op_error() - Get the error code.

 (2) afs_op_set_error() - Set the error code.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org