fs: btrfs: Implement btrfs_file_read()

This version of btrfs_file_read() has the following new features:
- Tries all mirrors
- More handling on unaligned size
- Better compressed extent handling
  The old implementation doesn't handle compressed extent with offset
  properly: we need to read out the whole compressed extent, then
  decompress the whole extent, and only then copy the requested part.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Marek Behún <marek.behun@nic.cz>