fbarray: make all fbarrays hidden files

fbarray stores its data in a shared file, which is not hidden.
This leads to polluting user's HOME directory with visible
files when running DPDK as non-root. Change fbarray to always
create hidden files by default.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>