ufs_inode_get{frag,block}(): consolidate success exits

These calling conventions are rudiments of pre-2.3 times; they
really need to be sanitized.  This is the first step; next
will be _always_ returning a block number, instead of this
"return a pointer to buffer_head, except when we get to the
actual data" crap.

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