fallocate: add --verbose, clean up usage()

Signed-off-by: Karel Zak <kzak@redhat.com>
fallocate: improve --dig-holes

 * don't use --length to specify hole size, always use stat.st_blksize

 * use --offset and --length to specify offset within the file (like
   another fallocate operations)

Signed-off-by: Karel Zak <kzak@redhat.com>