ext2fs: parallel bitmap loading

ext2fs: parallel bitmap loading

In our benchmarking for PiB size filesystem, pass5 takes
10446s to finish and 99.5% of time takes on reading bitmaps.

It makes sense to reading bitmaps using multiple threads,
a quickly benchmark show 10446s to 626s with 64 threads.

[ This has all of many bug fixes for rw_bitmaps.c from the original
  luster patch set collapsed into a single commit.   In addition it has
  the new ext2fs_rw_bitmaps() api proposed by Ted. ]

Signed-off-by: Wang Shilong <wshilong@ddn.com>
Signed-off-by: Saranya Muruganandam <saranyamohan@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>