VFS: Remove detached-dentry counter from shrink_dcache_for_umount_subtree()

Remove the detached-dentry counter from shrink_dcache_for_umount_subtree() as
the value it computes is no longer used as of commit
312d3ca8

 which made the nr_dentry counters
summed per-CPU rather than global atomic.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>