vm_reserv: fix zero-boundary error

Handle specially the boundary==0 case of vm_reserv_reclaim_config,
by turning off boundary adjustment in that case.

Reviewed by: alc
Tested by: pho, madpilot

(cherry picked from commit 49fd2d51f07fb29af3fd239f83f249c00cd84123)
(cherry picked from commit dd8ea1c755b3cc2c9c4eaa8611a97ac8f0f9b755)