filefrag: don't print extent header on bmap fallback

filefrag: don't print extent header on bmap fallback

The extent list header gets printed before we fall back to bmap:

# filefrag -v /mnt/test/bar
Filesystem type is: 58465342
File size of /mnt/test/bar is 12288 (3 blocks, blocksize 4096)
 ext logical physical expected length flags <---- HERE
Discontinuity: Block 2 is at 17 (was 16)
/mnt/test/bar: 2 extents found

so delay printing it until we know fiemap is working.

(though ideally it'd be nice to have the same verbose output
regardless of the interface we used, I think).

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>