nfs: Fix showing truncated fsid/dev in, /proc/net/nfsfs/volumes

nfs: Fix showing truncated fsid/dev in, /proc/net/nfsfs/volumes

A truncated fsid showing from /proc/fs/nfsfs/volumes as,
NV SERVER   PORT DEV     FSID              FSC
v4 c0a80881  801 0:43    34931f044c2a439b  no

It should be as,
NV SERVER   PORT DEV          FSID                              FSC
v4 c0a80881  801 0:43         34931f044c2a439b:954c5d830fa4be8c no

The max buffer length for storing "%llx:%llx" format should be
 16 + 1 + 16 + 1 = 34 (16 for %llx, 1 for ':', 1 for '\0').

Also, for storing "%u:%u" of MAJOR() and MINOR() should be
 8 + 1 + 3 + 1 = 13 (8 for 2^24, 1 for ':', 3 for 2^8, 1 for '\0').

v2, add comments for dev/fsid buffer and use sizeof in snprintf.

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>