3d_armor_sfinv doesn't check current formspec page

[Skamiz]

As a result the players inventory is being updated even when the armor tab isn't selected.
This interferes with other inventory mods if they don't work 100% through sfinv callbacks.

This obviously isn't much of an issue when the update is triggered through un/equipping of armor pieces, but the issue is, that the update can also be caused by items being moved into/out of the currently selected hotbar slot.