coredump: add COREDUMP_SPARSE to the coredump socket protocol

A coredump with a lot of unpopulated mappings sends useless amounts of
zero data to userspace. This is nonsensical. While __dump_skip() can
seek over them when the target is a regular file a socket cannot do
this. COREDUMP_RECORDS put the zeroes in records but it didn't get rid
of them.

Add a COREDUMP_SPARSE feature bit and a COREDUMP_RECORD_ZERO record
type. A zero record is a bare header that tells userspace how many zero
bytes were skipped.

So a hole crosses the socket as one header no matter how long it is. The
coredump server can recreate this sparsely. Zero records only exist
inside a record stream. COREDUMP_SPARSE requires COREDUMP_RECORDS.

Link: https://patch.msgid.link/20260820-work-coredump-sparse-v2-14-ba32dd718c51@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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