sysctl: don't loop when EAGAIN is returned

[tuexen]

Summary

When userland_sysctl() experiences EAGAIN, don't loop internally but leave retrying to the caller.

Looping was a problem in case of using tcpsso(8) to switch a TCP stack where the acceptance check of the new TCP stack results in EAGAIN.

MFC to: stable/15

[glebius]

Please include in the commit log references to the original phk's commit hashes and maybe our discussion in D60281.

At least state that this change is direct revert of 7a69d9230f834f562d2c5c36c837e67eacdf5ab3 from 1996.