Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r243245:

Add UFS writesuspension mechanism, designed to allow userland processes
to modify on-disk metadata for filesystems mounted for write.

Reviewed by: kib, mckusick
Sponsored by: FreeBSD Foundation

MFC r243247:

Add change missed in 243245.

MFC r243250:

Fix build of kdump(1).

MFC r243254:

Fix build on powerpc.

Reviewed by: nwhitehorn

MFC r243305 by marius:

Fix build after r243245.

Submitted by: trasz

MFC r243339 by kib:

Fix module build after r243245.

Add UFS writesuspension mechanism, designed to allow userland processes
to modify on-disk metadata for filesystems mounted for write.

Reviewed by: kib, mckusick
Sponsored by: FreeBSD Foundation