zfs: make zfs_vfs_held() definition consistent with declaration

Noticed while attempting to change 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.

Obtained from: https://github.com/openzfs/zfs/commit/62cc9d4f6
Reviewed by: jhb
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D39753