vm_page_free_invalid(): Relax the xbusy assertion.

vm_page_assert_xbusied() asserts that the busying thread is the current
thread. For some uses of vm_page_free_invalid() (e.g., error handling
in vnode_pager_generic_getpages_done()), this condition might not hold.

Reported by: Jenkins via trasz
Reviewed by: chs, kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D25828