9pfs: don't use AT_EMPTY_PATH in local_set_cred_passthrough()

9pfs: don't use AT_EMPTY_PATH in local_set_cred_passthrough()

The name argument can never be an empty string, and dirfd always point to
the containing directory of the file name. AT_EMPTY_PATH is hence useless
here. Also it breaks build with glibc version 2.13 and older.

It is actually an oversight of a previous tentative patch to implement this
function. We can safely drop it.

Reported-by: Mark Cave-Ayland <[email protected]>
Signed-off-by: Greg Kurz <[email protected]>
Tested-by: Mark Cave-Ayland <[email protected]>
Reviewed-by: Eric Blake <[email protected]>