fib_algo(4): Lower level of algorithm switching messages to LOG_INFO

Otherwise, with the default flm_debug_level of LOG_NOTICE, it's rather
easy to trigger debug messages such as:
[fib_algo] inet.0 (bsearch4#18) rebuild_fd_flm: switching algo to
radix4_lockless

Also, the "severity" of these events generally only justifies LOG_INFO
and not LOG_NOTICE.

Reviewed by: melifaro

(cherry picked from commit ed81a15517b8a8f587fd7282c3690513bb798242)