jfs: remove xtLookupList()

xtLookupList() was a more generalized version of xtLookup() with a
nastier interface.  Its only caller, extHint(), is actually better
suited to using xtLookup() than xtLookupList().  This also lets us
remove the definition of lxd_t, an obnoxious packed structure that was
only used in-memory.

Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>