gNFS: NFS daemon is limiting IOs to 64KB

Problem:
Gluster NFS server is hard-coding the max rsize/wsize to 64KB
which is very less for NFS running over 10GE NIC. The existing
options nfs.read-size, nfs.write-size are not working as
expected.

FIX:
Make the options nfs.read-size (for rsize) and nfs.write-size
(for wsize) work to tune the NFS I/O size. Value range would
be 4KB(Min)-64KB(Default)-1MB(max).

NB: Credit to "Richard Wareing" for catching it.

Change-Id: I2754ecb0975692304308be8bcf496c713355f1c8
BUG: 1009223
Signed-off-by: Santosh Kumar Pradhan <spradhan@redhat.com>
Reviewed-on: http://review.gluster.org/5964
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>