fincore: do not fall back to mincore if cachestat fails with EPERM

fincore: do not fall back to mincore if cachestat fails with EPERM

cachestat() and mincore() both require that the tested file is
(potentially) writable by the current user. If this permission check
fails, cachestat() will return EPERM while mincore() will simply mark
all pages as resident in core, as a proper EPERM would violate its API
contract. But when cachestat() fails with EPERM we know that mincore()
will not return real data, so instead show an error message.

Reported-by: Christian Hesse <mail@eworm.de>
Signed-off-by: Thomas Weißschuh <thomas@t-8ch.de>