statfs(2): allow to interrupt busying

There are probably more places which could benefit from allowing to
interrupt vfs_busy() calls at syscalls top level.

Requested by: Peter Eriksson <pen@lysator.liu.se>
Reviewed by: jah, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D58477