btrfs: headers cleanup to remove unnecessary local includes

btrfs: headers cleanup to remove unnecessary local includes

[BUG]
When I tried to remove btrfs_bio::fs_info and use btrfs_bio::inode to
grab the fs_info, the header "btrfs_inode.h" is needed to access the
full btrfs_inode structure.

Then btrfs will fail to compile.

[CAUSE]
There is a recursive including chain:

  "bio.h" -> "btrfs_inode.h" -> "extent_map.h" -> "compression.h" ->
  "bio.h"

That recursive including is causing problems for btrfs.

[ENHANCEMENT]
To reduce the risk of recursive including:

- Remove unnecessary local includes from btrfs headers
  Either the included header is pulled in by other headers, or is
  completely unnecessary.

- Remove btrfs local includes if the header only requires a pointer
  In that case let the implementing C file to pull the required header.

  This is especially important for headers like "btrfs_inode.h" which
  pulls in a lot of other btrfs headers, thus it's a mine field of
  recursive including.

- Remove unnecessary temporary structure definition
  Either if we have included the header defining the structure, or
  completely unused.

Now including "btrfs_inode.h" inside "bio.h" is completely fine,
although "btrfs_inode.h" still includes "extent_map.h", but that header
only includes "fs.h", no more chain back to "bio.h".

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>