drop_nlink() is the API function to decrease the link count of an inode.
However, at a place the control filesystem used the decrement operator
on i_nlink directly. Fix this.

Cc: Anand Avati <avati@gluster.com>
Signed-off-by: NCsaba Henk <csaba@gluster.com>
Signed-off-by: NMiklos Szeredi <mszeredi@suse.cz>