iflib: Improve return values of interrupt handlers.

iflib: Improve return values of interrupt handlers.

iflib was returning FILTER_HANDLED, in cases where FILTER_STRAY was more
correct. This potentially caused issues with shared legacy interrupts.

Driver filters returning FILTER_STRAY are now properly handled.

Submitted by:	Augustin Cavalier <waddlesplash@gmail.com>
Reviewed by:	marius, gallatin
Obtained from:	Haiku (a84bb9, 4947d1)
MFC after:	1 week
Sponsored by:	Limelight Networks
Differential Revision:	https://reviews.freebsd.org/D19201