fuse-bridge: enable --fopen-keep-cache based on FUSE_AUTO_INVAL_DATA.

If kernel supports FUSE_AUTO_INVAL_DATA then it is safe(r) to turn on
--fopen-keep-cache mode by default. Users report significant improvement
in perf by enabling the mode.

Change-Id: Icf9df4b7b43950d7e25302d9c2a1a7d14571a9a9
BUG: 990744
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/5770
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>