btrfs: move misplaced comment of btrfs_path::keep_locks

Commit 925baedd ("Btrfs: Start btree concurrency work.") added the
comment for the field keep_locks. This got moved later but without the
comment, so move it to the right place and fix the comment style.

Signed-off-by: Sun YangKai <sunk67188@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>