LINUX: Use correct type of error in flock code

LINUX: Use correct type of error in flock code

The flock code gets a kernel error code from afs_posix_lock_file()
but then passes this error code through afs_convert_code (which sees
that it is already negative, and returns EIO). Instead, we should just
return afs_posix_lock_file()'s code direct to our caller.

Change-Id: I28bbc2dc8535d28a4e9e9f564392fd4cbfa76569
Reviewed-on: http://gerrit.openafs.org/2534
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>