f2fs: fix to adjust appropriate length for fiemap

If user give a file size as "length" parameter for fiemap
operations, but if this size is non-block size aligned,
it will show 2 segments fiemap results even this whole file
is contiguous on disk, such as the following results:

 ./f2fs_io fiemap 0 19034 ylog/analyzer.py
Fiemap: offset = 0 len = 19034
        logical addr.    physical addr.   length           flags
0       0000000000000000 0000000020baa000 0000000000004000 00001000
1       0000000000004000 0000000020bae000 0000000000001000 00001001

after this patch:
./f2fs_io fiemap 0 19034 ylog/analyzer.py
Fiemap: offset = 0 len = 19034
    logical addr.    physical addr.   length           flags
0    0000000000000000 00000000315f3000 0000000000005000 00001001

Signed-off-by: Zhiguo Niu <zhiguo.niu@unisoc.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>