MFC	r262346:
ext2fs: fully enable ext4 read-only support.

The ext4 developers tend to tag Ext4-specific flags as
"incompatible" even when such features are not relevant for
read-only support.  This is a consequence of the process
though which this filesystem is implemented without design
and the fact that some new features are not extensible to
ext2/3.

Organize the features according to what we support and sort
them so that we can now read-only mount filesystems with
some features that may be found in newly formatted ext4 fs.

Submitted by:	Zheng Liu
MFC r257472

Rate limit (to once per minute) "Listen queue overflow" message in
sonewconn().
MFC: r262296

Match the correct variable to the variable description.

PR:		121173
Submitted by:	Thomas Mueller <tmueller at sysgo.com>
MFC r262142:

   In ue_attach_post_task(), initialize curvnet to vnet0 before calling if_attach().
   Before this patch, curvnet was NULL.
   When the VIMAGE kernel option is enabled, this eliminates
   kernel panics when USB ethernet devices are plugged in.

PR: 183835
Submitted by: Hiroo Oono <hiroo.ono at gmail dot com>
[automated] Merge in upstream-github/stable/9