Linux: use filemap_range_has_page()

As of the 4.13 kernel filemap_range_has_page() can be used to
check if there is a page mapped in a given file range. When
available this interface should be used which eliminates the
need for the zp->z_is_mapped boolean.

Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #14493