fuse: pass the mountflags to older (< 2.6.21) kernels

The change for Bug 853895 fixed mounting volumes read-only for recent
kernels. Older kernels fail the first mount() syscall, and the second
mount() did not add the 'mountflags'.

Full analysis and a description for reproducing is in the bugreport. The
test included in http://review.gluster.org/4163 would have caught this
problem when the tests are executed on RHEL-5 or similar systems.

Change-Id: Ifa6cd6476bf1939e009142ed4ed1b53b9c3171bd
Master-Branch-Change-ID: I440591344a6a5af7b2018e37a2a1fda9de8b5ab2
Bug: 996768
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/5752
Reviewed-by: Shishir Gowda <sgowda@redhat.com>
Reviewed-by: Raghavendra Bhat <raghavendra@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>