pid1,nspawn: raise default RLIMIT_MEMLOCK to 8M

pid1,nspawn: raise default RLIMIT_MEMLOCK to 8M

This mirrors a similar check in Linux kernel 5.16
(9dcc38e2813e0cd3b195940c98b181ce6ede8f20) that raised the
RLIMIT_MEMLOCK to 8M.

This change does two things: raise the default limit for nspawn
containers (where we try to mimic closely what the kernel does), and
bump it when running on old kernels which still have the lower setting.

Fixes: #16300
See: https://lwn.net/Articles/876288/