bypasscache: do errors correctly

bypasscache: do errors correctly

do set errors when we bomb out early
do not unlock and return early when we happen to do a correct zero
length read
do set errors the kernel can deal with if we're feeding a page routine

Reviewed-on: http://gerrit.openafs.org/5554
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 28a159b2bfff7ad450dce612601a0286d4f91df5)

Change-Id: I4f2e1b50a3c0856a08281e0ffbfaa7cbb71db2c4
Reviewed-on: http://gerrit.openafs.org/5716
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>