This patch adds export of generic_forget_inode()

if sb->drop_inode method is set, than it's called in iput_final().
But it's impossible to call neither generic_drop_inode(), nor 
generic_forget_inode() inside this handler. Only generic_delete_inode() 
is accessiable.

Signed-Off-By: Kirill Korotaev <dev@sw.ru>

Kirill