filefrag: fix report of number of extents

filefrag: fix report of number of extents

filefrag has several bugs:

1.
$ touch f1
$ filefrag f1
f1: 1 extent found  ----> bug!
$ filefrag -v f1
Filesystem type is: ef53
File size of f1 is 0 (0 blocks, blocksize 4096)
f1: 0 extents found

2.
$ truncate -s 1m f2
$ filefrag f2
f2: 1 extent found  ----> bug!
$ filefrag -v f2
Filesystem type is: ef53
File size of f2 is 1048576 (256 blocks, blocksize 4096)
f2: 0 extents found

3.
$ for i in `seq 11 -2 0`; do dd if=/dev/zero of=f4 bs=4k count=1 seek=$i conv=notrunc oflag=sync &>/dev/null; done
$ ll f4
-rw-r--r-- 1 root root 49152 Jun  9 15:09 f4
$ filefrag f4
f4: 7 extents found
$ filefrag -v f4
Filesystem type is: ef53
File size of f4 is 49152 (12 blocks, blocksize 4096)
 ext logical physical expected length flags
   0       1  1109993               1
   1       3  1109992  1109994      1
   2       5  1109991  1109993      1
   3       7  1109990  1109992      1
   4       9  1109989  1109991      1
   5      11  1108207  1109990      1 eof
f4: 7 extents found  -----------------------> but we only have 6 extents, bug!

All of these bugs come from the fact that we've made a mistake on
calculating total extents:

o   we set 1 as default for 'total extents', and this will report 1
    extent found even when we don't get any extent from fiemap.
o   if our first extent does not start from 0(logical addr), total
    extents will be one more than what it should be.

Addresses-Red-Hat-Bugzilla: #840848

Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Liu Bo <liubo2009@cn.fujitsu.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>