revert r272702: wrong (earlier) change was committed
l2arc_write_buffers: reduce headroom value

FreeBSD has ARC_BUFC_NUMMETADATALISTS metadata lists and ARC_BUFC_NUMDATALISTS
data lists (currently both are 16) while illumos has just a single list
of each kind.

headroom determines how much data is scanned on a single list
during each run of the l2arc feed thread.
Because FreeBSD has more lists we proportionally decrease the limit.

Reviewed by:	Brendan Gregg (earlier version)
MFC after:	2 weeks
Sponsored by:	HybridCluster
Implement the FLUSH operation in the virtio-block emulation.

This gets rid of the following error message during FreeBSD guest bootup:
"vtbd0: hard error cmd=flush fsbn 0"

Reported by:	rodrigc
Reviewed by:	grehan
Add driver for Synopsys DesignWare Mobile Storage Host Controller.

Sponsored by:	DARPA, AFRL
Since introducing the extra mapping in r250103 for architectural performance
events we have actually counted 'Branch Instruction Retired' when people
asked for 'Unhalted core cycles' using the 'unhalted-core-cycles' event mask
mnemonic.

Reviewed by:	jimharris
Discussed with:	gnn, rwatson
MFC after:	3 days
Sponsored by:	DARPA/AFRL
Allow vt(4) to disable terminal bell with `sysctl kern.vt.bell_enable=0`,
similar as syscons(4) do.

Submitted by:	Tiwei Bie <btw@mail.ustc.edu.cn>
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Support the Vodafone R215 LET USB dongle, which is apparently a rebadged
E5372 with different product IDs.

Interestingly, the standard E5372 IDs (12d1:1506) are currently listed in
u3g.c and are the same as the E3131.  However, the R215/E5372 is an NCM
device and works well with cdce(4) whereas the E3131 isn't.  More work
may be needed to better identify the other device IDs.

MFC after:	1 week
Remove call to access(2) which didn't serve any purpose, and make it more
tolerant to errors.

MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Make kern.nswbuf tunable from loader.

MFC after:	1 week
cxgbe/tom: don't leak resources tied to an active open request that
cannot be sent to the chip because a prerequisite L2 resolution
failed.

Submitted by:	Hariprasad at chelsio dot com (original version)
MFC after:	2 weeks.
Implement PLPMTUD blackhole detection (RFC 4821), inspired by code
from xnu sources.  If we encounter a network where ICMP is blocked
the Needs Frag indicator may not propagate back to us.  Attempt to
downshift the mss once to a preconfigured value.

Default this feature to off for now while we do not have a full PLPMTUD
implementation in our stack.

Adds the following new sysctl's for control:
net.inet.tcp.pmtud_blackhole_detection -- turns on/off this feature
net.inet.tcp.pmtud_blackhole_mss       -- mss to try for ipv4
net.inet.tcp.v6pmtud_blackhole_mss     -- mss to try for ipv6

Adds the following new sysctl's for monitoring:
-- Number of times the code was activated to attempt a mss downshift
net.inet.tcp.pmtud_blackhole_activated
-- Number of times the blackhole mss was used in an attempt to downshift
net.inet.tcp.pmtud_blackhole_min_activated
-- Number of times that we failed to connect after we downshifted the mss
net.inet.tcp.pmtud_blackhole_failed

Phabricator:	https://reviews.freebsd.org/D506
Reviewed by:	rpaulo bz
MFC after:	2 weeks
Relnotes:	yes
Sponsored by:	Limelight Networks