btrfs: add workspace manager initialization for zstd

This involves:

- Add zstd_alloc_workspace_manager() and zstd_free_workspace_manager()
  Those two functions will accept an fs_info pointer, and alloc/free
  fs_info->compr_wsm[BTRFS_COMPRESS_ZSTD] pointer.

- Add btrfs_alloc_compress_wsm() and btrfs_free_compress_wsm()
  Those are helpers allocating the workspace managers for all
  algorithms.
  For now only zstd is supported, and the timing is a little unusual,
  the btrfs_alloc_compress_wsm() should only be called after the
  sectorsize being initialized.

  Meanwhile btrfs_free_fs_info_compress() is called in
  btrfs_free_fs_info().

- Move the definition of btrfs_compression_type to "fs.h"
  The reason is that "compression.h" has already included "fs.h", thus
  we can not just include "compression.h" to get the definition of
  BTRFS_NR_COMPRESS_TYPES to define fs_info::compr_wsm[].

For now the per-fs zstd workspace manager won't really have any effect,
and all compression is still going through the global workspace manager.

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