popcorn should not use ksys_{brk,mremap,madvise,mprotect}

[bxatnarf]

ksys_brk, ksys_mremap, ksys_madvise, and ksys_mprotect are not supposed to be globally accessible but are because popcorn uses them. For example: https://github.com/ssrg-vt/popcorn-kernel/blob/e44428e9aa4f9656322749dc94834f59659eeb3b/kernel/popcorn/vma_server.c#L502