f2fs: fix bugs and simplify codes of f2fs_fiemap

fix bugs:
1. len could be updated incorrectly when start+len is beyond isize.
2. If there is a hole consisting of more than two blocks, it could
   fail to add FIEMAP_EXTENT_LAST flag for the last extent.
3. If there is an extent beyond isize, when we search extents in a range
   that ends at isize, it will also return the extent beyond isize,
   which is outside the range.

Signed-off-by: Fan li <fanofcode.li@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>