fsstress: print syncfs() return value in verbose mode

We aren't logging the syncfs() return value in case we are running in
verbose mode, which is useful and it would help me immediately figuring
out it was failing in a problem I was debugging with btrfs.

So log its return value, just like we do for every other fsstress command.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>