reiserfs: truncate blocks not used by a write

It can happen that write does not use all the blocks allocated in
write_begin either because of some filesystem error (like ENOSPC) or
because page with data to write has been removed from memory.  We truncate
these blocks so that we don't have dangling blocks beyond i_size.

Cc: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>