Btrfs: rename and export get_chunk_map

The Btrfs swap code is going to need it, so give it a btrfs_ prefix and
make it non-static.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.h

@@ -462,6 +462,8 @@ unsigned long btrfs_full_stripe_len(struct btrfs_fs_info *fs_info,
 int btrfs_finish_chunk_alloc(struct btrfs_trans_handle *trans,
 			     u64 chunk_offset, u64 chunk_size);
 int btrfs_remove_chunk(struct btrfs_trans_handle *trans, u64 chunk_offset);
+struct extent_map *btrfs_get_chunk_map(struct btrfs_fs_info *fs_info,
+				       u64 logical, u64 length);
 
 static inline void btrfs_dev_stat_inc(struct btrfs_device *dev,
 				      int index)