nfs/super.c: local functions should be static

nfs/super.c: local functions should be static

commit ae50c0b5 "pnfs: client stats" added additional information to
the output of /proc/self/mountstats. The new functions introduced are
only used in this file and should be marked static.

If CONFIG_NFS_V4_1 is not defined, empty stub functions are used.  If
CONFIG_NFS_V4 is not defined these stub functions are not used at all.
Adding static for the functions results in compile warnings:

fs/nfs/super.c:743: warning: 'show_sessions' defined but not used
fs/nfs/super.c:756: warning: 'show_pnfs' defined but not used

Fix this by adding a #ifdef CONFIG_NFS_V4 guard around the two
show_ functions.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>