Writing of inode holding unallocated space info was broken because we first
cleared the buffer and after that checked whether it contains a tag meaning the
block holds unallocated space information.  Fix the problem by checking
appropriate in memory flag instead.

Also cleanup the function a bit along the way - most importantly lock buffer
when modifying its contents, check for buffer_write_io_error instead of
!buffer_uptodate, etc..
Signed-off-by: NJan Kara <jack@suse.cz>