glusterfsd: Use dynamic volfile buffer

Glusterfsd used a fixed buffer to store volfiles fetched via getspec.
This caused problems with large volfiles. Changining this to a dynamic
buffer allows large volfiles to be loaded in memory.

Change-Id: I40236dcb1c37b9a0136dfb5231cafabb3d4f00dc
BUG: 986100
Signed-off-by: Kaushal M <kaushal@redhat.com>
Reviewed-on: http://review.gluster.org/5373
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>