glusterfsd: Use dynamic volfile buffer

Backport of
 fa78792 glusterfsd: Use dynamic volfile buffer
from the master branch.

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.

BUG: 986100
Change-Id: I9ec86c09a00bc84fef7a0da05bbeb6b3e07d5146
Signed-off-by: Kaushal M <kaushal@redhat.com>
Reviewed-on: http://review.gluster.org/5431
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>