FreeBSD: make zfs_vfs_held() definition consistent with declaration

Noticed while attempting to change FreeBSD's boolean_t into an actual
bool: in include/sys/zfs_ioctl_impl.h, zfs_vfs_held() is declared to
return a boolean_t, but in module/os/freebsd/zfs/zfs_ioctl_os.c it is
defined to return an int. Make the definition match the declaration.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Dimitry Andric <dimitry@andric.com>
Closes #14776