fs/nfs: fix new compiler warning about boolean in switch

The brand new GCC 5.1.0 warns by default on using a boolean in the
switch condition. This results in the following warning:

fs/nfs/nfs4proc.c: In function 'nfs4_proc_get_rootfh':
fs/nfs/nfs4proc.c:3100:10: warning: switch condition has boolean value [-Wswitch-bool]
  switch (auth_probe) {
          ^

This code was obviously using switch to make use of the fall-through
semantics (without the usual comment, though).
Rewrite that code using if statements to avoid the warning and make
the code a bit more readable on the way.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>