[PATCH] NFSv4: fix mount segfault on errors returned that are < -1000

It turns out that nfs4_proc_get_root() may return raw NFSv4 errors instead of
mapping them to kernel errors.  Problem spotted by Neil Horman
<nhorman@tuxdriver.com>

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>