fsstress: unify verbose output format

Wrap offset, length within '[]' and remove meanless 't' in
do_fallocate().

Signed-off-by: Shiyang Ruan <ruansy.fnst@fujitsu.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>