filefrag: fix fm_start in filefrag_fiemap loop

filefrag: fix fm_start in filefrag_fiemap loop

When used with -v and the targeted file has more than 144
extents(double of the length of fm_extents array provided by buf),
filefrag_fiemap loops and calls fiemap ioctl() multiple times to
calculate the actual number of extents in a file. Each call to fiemap
ioctl() uses fm_start as the starting logical offset. The patch fixes
fm_start in each loop( except for the first one) and makes the extent
calculation correct for files with more that 144 extents.

To produce the problem, first run filefrag -v on a highly fragmented
file.  Then change the buf size in filefrag_fiemap to make it large
enough to have all the extent mapped in a single loop and run filefrag
-v after recompiling.  The former will produce a much smaller extent
count because of the false fm_start used in the loop. And the two will
produce different extent output since the 145th extent.

Signed-off-by: Peng Tao <bergwolf@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>