nilfs2: add helper functions to delete blocks from dat file

This adds delete functions for data blocks of metadata files using bitmap
based allocator.  nilfs_palloc_delete_entry_block() deletes an entry block
(e.g.  block storing dat entries), and nilfs_palloc_delete_bitmap_block()
deletes a bitmap block, respectively.

These helpers are intended to be used in the successive change on
deallocator of block addresses ("nilfs2: free unused dat file blocks
during garbage collection").

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>