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

module/os/linux/zfs/zfs_ctldir.c

@@ -498,7 +498,9 @@ zfsctl_inode_alloc(zfsvfs_t *zfsvfs, uint64_t id,
 	zp->z_atime_dirty = B_FALSE;
 	zp->z_zn_prefetch = B_FALSE;
 	zp->z_is_sa = B_FALSE;
+#if !defined(HAVE_FILEMAP_RANGE_HAS_PAGE)
 	zp->z_is_mapped = B_FALSE;
+#endif
 	zp->z_is_ctldir = B_TRUE;
 	zp->z_sa_hdl = NULL;
 	zp->z_blksz = 0;