NFS: Squelch compiler warning in nfs_add_server_stats()

Clean up:

fs/nfs/iostat.h: In function ‘nfs_add_server_stats’:
fs/nfs/iostat.h:41: warning: comparison between signed and unsigned integer expressions
fs/nfs/iostat.h:41: warning: comparison between signed and unsigned integer expressions
fs/nfs/iostat.h:41: warning: comparison between signed and unsigned integer expressions
fs/nfs/iostat.h:41: warning: comparison between signed and unsigned integer expressions

Commit fce22848 replaced the open-coded per-cpu logic in several
functions in fs/nfs/iostat.h with a single invocation of
this_cpu_ptr().  This macro assumes its second argument is signed,
not unsigned.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>