Saner handling of "lseek should fail with ESPIPE" - gets rid of

magical no_llseek thing and makes checks consistent.  In particular,
ad-hoc "can we do splice via internal pipe" checks got saner (and
somewhat more permissive, which is what Jason had been after, AFAICT)

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
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