linker_kldload_busy(): allow recursion

[kib]

Summary

Some drivers recursively loads modules by explicit calls to kldload
during initialization, which might occur during kldload.

PR:     259748

[markj]

The change looks ok, but after re-reading D30456 I wonder if busying should be done in sys_kldload() and sys_kldunload() instead. If the kernel initiates an unload at the "wrong" time then it's a bug in the initiator.