Revision 323314

Audit userspace geom code for leaking memory to disk

Any geom class using g_metadata_store, as well as geom_virstor which
duplicated g_metadata_store internally, would dump sectorsize - mdsize bytes
of userspace memory following the metadata block stored. This is most or all
geom classes (gcache, gconcat, geli, gjournal, glabel, gmirror, gmultipath,
graid3, gshsec, gstripe, and geom_virstor).

PR:		222077 (comment #3)
Reported by:	Maxim Khitrov <max AT mxcrypt.com>
Reviewed by:	des
Security:	yes
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D12269