fsstress: Add fallocate zero range operation

fsstress: Add fallocate zero range operation

This commit adds fzero operation support for fsstress, which is meant to
exercise fallocate FALLOC_FL_ZERO_RANGE support.

Also reorganise the common fallocate code into a single do_fallocate()
function and use flags use the right mode.

Also in order to make more obvious which fallocate mode fsstress is
testing translate fallocate flags into human readable strings.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>