nilfs2: remove buffer locking from btree code

lock_buffer() and unlock_buffer() uses in btree.c are eliminable
because btree functions gain buffer heads through nilfs_btnode_get(),
which never returns an on-the-fly buffer.

Although nilfs_clear_dirty_page() and nilfs_copy_back_pages() in
nilfs_commit_gcdat_inode() juggle btree node buffers of DAT, this is
safe because these operations are protected by a log writer lock or
the metadata file semaphore of DAT.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>