ufs_inode_get{frag,block}(): get rid of retries

We are holding ->truncate_mutex, so nobody else can alter our
block pointers.  Rechecks/retries were needed back when we
only held BKL there, and had to cope with write_begin/writepage
and writepage/truncate races.  Can't happen anymore...

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