ufs: free excessive blocks upon ->write_begin() failure/short copy

Broken in "[PATCH] ufs: truncate should allocate block for last byte";
all way back in 2006.  ufs_setattr() hadn't been the only user of
vmtruncate() and eliminating ->truncate() method required corrections
in a bunch of places.  Eventually those places had migrated into
->write_begin() failure exit and ->write_end() after short copy...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>