NFS: Fix use of nfs_attr_use_mounted_on_fileid()

This function call was being optimized out during nfs_fhget(), leading
to situations where we have a valid fileid but still want to use the
mounted_on_fileid.  For example, imagine we have our server configured
like this:

server % df
Filesystem      Size  Used Avail Use% Mounted on
/dev/vda1       9.1G  6.5G  1.9G  78% /
/dev/vdb1       487M  2.3M  456M   1% /exports
/dev/vdc1       487M  2.3M  456M   1% /exports/vol1
/dev/vdd1       487M  2.3M  456M   1% /exports/vol2

If our client mounts /exports and tries to do a "chown -R" across the
entire mountpoint, we will get a nasty message warning us about a circular
directory structure.  Running chown with strace tells me that each directory
has the same device and inode number:

newfstatat(AT_FDCWD, "/nfs/", {st_dev=makedev(0, 38), st_ino=2, ...}) = 0
newfstatat(4, "vol1", {st_dev=makedev(0, 38), st_ino=2, ...}) = 0
newfstatat(4, "vol2", {st_dev=makedev(0, 38), st_ino=2, ...}) = 0

With this patch the mounted_on_fileid values are used for st_ino, so the
directory loop warning isn't reported.

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>