[Bug 220032] [if_alc] sys/dev/alc/if_alc.c: sleep-under-mutex bugs

[Jia-Ju Bai]

The alc driver may sleep under a mutex, and the function call paths in file
"sys/dev/alc/if_alc.c" in FreeBSD 11.0 is:
alc_resume [line 2648: acquire the mutex]
   alc_init_locked [line 2664]
     alc_init_rx_ring [line 3911]
       alc_newbuf [line 4469]
         bus_dmamap_load_mbuf_sg(BUS_DMA_WAITOK) [line 3472] --> may sleep

alc_int_task [line 3330: acquire the mutex]
   alc_start_locked [line 3372]
     alc_encap [line 2942]
       bus_dmamap_load_mbuf_sg(BUS_DMA_WAITOK) [line 2780] --> may sleep

The possible fix of these bugs is to set the last parameter in
bus_dmamap_load_mbuf_sg to "BUS_DMA_NOWAIT".

This bug is found by a static analysis tool written by myself, and it is
checked by my review of the FreeBSD code.

By the way, I am a freshman in developing FreeBSD drivers, and I am willing to
submit a patch. But I do not know how to write and submit a patch, and where to
submit, so I am looking forward to useful advice :)

Jia-Ju Bai