This patch fixes dcache race between shrink_dcache_XXX functions
and dput().

Example race scenario:

	CPU 0				CPU 1
umount /dev/sda1
generic_shutdown_super          shrink_dcache_memory()
shrink_dcache_parent            dput dentry
select_parent                   prune_one_dentry()
                                 <<<< child is dead, locks are released,
                                   but parent is still referenced!!! >>>>

skip dentry->parent,
since it's d_count > 0

message: BUSY inodes after umount...
                                 <<< parent is left on dentry_unused
				   list, referencing freed super block

We faced these messages about busy inodes constantly after some stress 
testing with mount/umount operations parrallel with some other activity.
This patch helped the problem.

The patch was heavilly tested on 2.6.8 during 2 months,
this forward-ported version boots and works ok as well.

Signed-Off-By: Kirill Korotaev <dev@sw.ru>
Signed-Off-By: Andrey Savochkin <saw@saw.sw.com.sg>
Signed-Off-By: Dmitry Mishin <dim@sw.ru>