glusterd: validate performance.nfs.* option values during volume set stage

PROBLEM:

performance.nfs.* option values (which are of type boolean) are
not validated during the stage phase of 'volume set'.
The result - nfs graph generation fails during commit phase,
AFTER the option and its (invalid) value have been placed in
volinfo->dict.

CAUSE:

nfsperfxl_option_handler() - the function that validates the values of
performance.nfs.* options - never receives the (key,value) pair that
needs to be set, for validation during 'volume set' stage.

FIX:

In build_nfs_graph(), copy the (mod_)dict containing the (option,value)
parameters into set_dict before attempting to build the client graph
for the volume on which the operation is being performed.

Of course, an easier way out would be to simply do a 'volume reset' and
pretend nothing wrong happened!

Change-Id: I56b17d0239d58a9e0b7798933a3c8451e2675b69
BUG: 949930
Signed-off-by: Krutika Dhananjay <kdhananj@redhat.com>
Reviewed-on: http://review.gluster.org/4814
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>