vfs: Add ioflag to VOP_DEALLOCATE(9)

The addition of ioflag allows callers passing
IO_SYNC/IO_DATASYNC/IO_DIRECT down to the file system implementation.
The vop_stddeallocate fallback implementation is updated to pass the
ioflag to the file system implementation. vn_deallocate(9) internally is
also changed to pass ioflag to the VOP_DEALLOCATE call.

Sponsored by: The FreeBSD Foundation
Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D31500