Always unlock pages when returning from writepage

Always unlock pages when returning from writepage

Writepage has a return path which returns an error with a locked
page. However, all returns that are not AOP_WRITEPAGE_ACTIVATE must
unlock their pages - using this codepath would leave a stray page lock,
which would eventually hang the machine.

The logic behind the -EIO return was also incorrect. In the Linux page
cache model, truncates simply reduce the size recorded in the inode. If
there are pages pending writeback then they may still have writepage() called
upon them - it's up to the writepage routine to discard the write
request (rather than returning an error)

Reviewed-on: http://gerrit.openafs.org/685
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>