Make linux(4) support splice(2) - kind of

[trasz]

Summary

Add a trivial linux(4) splice(2) implementation, which simply
returns EINVAL. Fixes grep (grep-3.1-2build1).

PR: kern/247219

[trasz]

Upload the right diff.

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Add arm64.

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(Tinderboxed.)

[emaste]

sys/compat/linux/linux_file.c

1774 ↗

(On Diff #74361)

so consumers handle EINVAL from splice but not ENOSYS?

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sys/compat/linux/linux_file.c

1774 ↗

(On Diff #74361)

Precisely. Well, the one I found: grep. But looking at splice(2) man page, consumers probably need to handle it anyway: with early kernels exactly one of the fds had to be a pipe, later both could be, then there's "EINVAL The target filesystem doesn't support splicing"...

[emaste]

Probably worth putting that EINVAL description in a comment by the return.