Move a racy assertion in filt_pipewrite().

Move a racy assertion in filt_pipewrite().

EVFILT_WRITE knotes for pipes live on the knlist for the other end of the
pipe.  Since they do not hold a reference on the corresponding file
structure, they may be removed from the knlist by pipeclose() while still
remaining active.  In this case, there is no knlist lock acquired before
filt_pipewrite() is called, so the assertion fails.

Fix the problem by first checking whether that end of the pipe has been
closed.  These checks are memory safe since the knote holds a reference
on one end of the pipe, and the pipe structure is not freed until both
ends are closed.  The checks are not racy since PIPE_EOF is never cleared
after being set, and pipe_present is never set back to PIPE_ACTIVE after
pipeclose() has been called.

PR:		235640
Reported and tested by:	pho
Reviewed by:	kib
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D19224