efirt(9): carefully destroy efi_lock

[kib]

Summary

efi_init() might return error after initializing the mutex, in which
case MOD_UNLOAD() is not processed, and the mutex is not destroyed.
Similarly, efi_uninit() skips any processing if efi_runtime was left as
NULL, leaving mutex not destroyed.

Initialize the mutex in MOD_LOAD case, and destroy in MOD_UNLOAD, also
handling errors.