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.

Reviewed-on: http://gerrit.openafs.org/2534
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 5abe706a7e237d3872db3638d947a14c935370d5)
Change-Id: I8e12c9804cc81c8c88f820f55abd4ce863525c9a
Reviewed-on: http://gerrit.openafs.org/3533