f2fs: stop iterating f2fs_map_block if hole exists

Let's avoid unnecessary f2fs_map_block calls to load extents.

 # f2fs_io fadvise willneed 0 4096 /data/local/tmp/test

  f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 386, start blkaddr = 0x34ac00, len = 0x1400, flags = 2,
  f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 5506, start blkaddr = 0x34c200, len = 0x1000, flags = 2,
  f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 9602, start blkaddr = 0x34d600, len = 0x1200, flags = 2,
  f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 14210, start blkaddr = 0x34ec00, len = 0x400, flags = 2,
  f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 15235, start blkaddr = 0x34f401, len = 0xbff, flags = 2,
  f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 18306, start blkaddr = 0x350200, len = 0x1200, flags = 2
  f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 22915, start blkaddr = 0x351601, len = 0xa7d, flags = 2
  f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 25600, start blkaddr = 0x351601, len = 0x0, flags = 0
  f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 25601, start blkaddr = 0x351601, len = 0x0, flags = 0
  f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 25602, start blkaddr = 0x351601, len = 0x0, flags = 0
  ...
  f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 1037188, start blkaddr = 0x351601, len = 0x0, flags = 0
  f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 1038206, start blkaddr = 0x351601, len = 0x0, flags = 0
  f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 1039224, start blkaddr = 0x351601, len = 0x0, flags = 0
  f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 2075548, start blkaddr = 0x351601, len = 0x0, flags = 0

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>