buffer device_printf...

On my arm64 board, various console messages get intermixed because of
SMP.  This makes messages harder to read than the could be.  One issue
is that device_printf uses two printf's, one for the device, and one
for the provided message..  This prevents PRINTF_BUFR_SIZE from combining
the two messages into a single write.

The review https://reviews.freebsd.org/D16690 makes device_printf use
an sbuf w/ a printf drain to effectively buffer these messages together.

previously:
Trying to mount root from ufs:/dev/ufs/rootfs [rw]...
mmc0:  Instruction Set Attributes 0 = <AES+PMULL,SHA1,SHA2,CRC32>
ACMD42 failed, RESULT: 4
 Instruction Set Attributes 1 = <>
mmc0:          Processor Features 0 = <AdvSIMD,Float,EL3 32,EL2 32,EL1 32,EL0 32>
Card at relative address 43690 failed to set bus width

w/ patch:
mmc0: ACMD42 failed, RESULT: 4
Trying to mount root from ufs:/dev/ufs/rootfs [rw]...
 Instruction Set Attributes 0 = <AES+PMULL,SHA1,SHA2,CRC32>
mmc0: Card at relative address 43690 failed to set bus width
 Instruction Set Attributes 1 = <>
         Processor Features 0 = <AdvSIMD,Float,EL3 32,EL2 32,EL1 32,EL0 32>

Comments?

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  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."