assorted variants of irqfd setup: convert to CLASS(fd)

assorted variants of irqfd setup: convert to CLASS(fd)

in all of those failure exits prior to fdget() are plain returns and
the only thing done after fdput() is (on failure exits) a kfree(),
which can be done before fdput() just fine.

NOTE: in acrn_irqfd_assign() 'fail:' failure exit is wrong for
eventfd_ctx_fileget() failure (we only want fdput() there) and once
we stop doing that, it doesn't need to check if eventfd is NULL or
ERR_PTR(...) there.

NOTE: in privcmd we move fdget() up before the allocation - more
to the point, before the copy_from_user() attempt.

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