Patch from Maneesh Soni <maneesh@in.ibm.com>

Turns out that sysfs is doing dget() on a zero-ref dentry.  That's a bug, but
dcache is no longer detecting it.

The check was removed because with lockless d_lookup, there can be cases when
d_lookup and dput are going on concurrently, If d_lookup happens earlier then
it may do dget() on a dentry for which dput() has decremented the ref count
to zero.  This race is handled by taking the per dentry lock and checking the
DCACHE_UNHASHED flag.

The patch open-codes that part of d_lookup(), and restores the BUG check in
dget().