ufs_inode_getblock(): pass index instead of 'fragment'

The value passed to ufs_inode_getblock() as the 3rd argument
had lower bits ignored; the upper bits were shifted down
and used and they actually make sense - those are _lower_ bits
of index in indirect block (i.e. they form the index within
a fragment within an indirect block).

Pass those as argument.  Upper bits of index (i.e. the number
of fragment within indirect block) will join them shortly.

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