staging/lustre/ldlm: Reinstate ldlm_enqueue_pack()

The function becomes used again with the next patch, so bring it back
from dead, only this time make it static.

Reverts: bf2a0333 ("staging/lustre/ldlm: Remove unused ldlm_enqueue_pack()")
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>