This patch fixes two bugs, which introduced by previous patches:

1) Missed "brelse"

2) Sometimes "baseblk" may be wrongly calculated, if i_size is equal to
   zero, which lead infinite cycle in "mpage_writepages".
Signed-off-by: NEvgeniy Dushistov <dushistov@mail.ru>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>