btrfs: fix unbalanced unlock of mapping_tree_lock

The error path of btrfs_get_chunk_map() releases
fs_info->mapping_tree_lock. But, it is taken and released in
btrfs_find_chunk_map(). So, there is no need to do so.

Fixes: 7dc66abb ("btrfs: use a dedicated data structure for chunk maps")
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>