Staging: p9auth: fix credential logic

current->uid is no longer allowed in the 2.6.29 kernel, so use
the proper credential api to be able to alter the uid and euid values.

Note, this now builds properly, hopefully still works properly, would be
good for someone to test it out...

Cc: Ashwin Ganti <ashwin.ganti@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>