client: xlist link dentries instead of set

client: xlist link dentries instead of set

This saves space and avoids unnecessary set logic. In particular, we no longer
need to do a heap allocation for each Dentry * in the std::set.

Before:

(gdb) print sizeof(Inode)
$1 = 1336
(gdb) print sizeof(Inode::dn_set)
$2 = 48

After:

(gdb) print sizeof(Inode)
$1 = 1360
(gdb) print sizeof(Inode::dentries)
$2 = 24

I'm not sure why the Inode size increased when the member size decreased (weird
padding by g++)? Anyway, we still get the benefit of no heap allocations for
the Dentry *s.

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>