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

include/sys/zfs_znode.h

@@ -188,7 +188,6 @@ typedef struct znode {
 	boolean_t	z_atime_dirty;	/* atime needs to be synced */
 	boolean_t	z_zn_prefetch;	/* Prefetch znodes? */
 	boolean_t	z_is_sa;	/* are we native sa? */
-	boolean_t	z_is_mapped;	/* are we mmap'ed */
 	boolean_t	z_is_ctldir;	/* are we .zfs entry */
 	boolean_t	z_suspended;	/* extra ref from a suspend? */
 	uint_t		z_blksz;	/* block size in bytes */