Fix lseek(SEEK_DATA/SEEK_HOLE) mmap consistency

When using lseek(2) to report data/holes memory mapped regions of
the file were ignored.  This could result in incorrect results.
To handle this zfs_holey_common() was updated to asynchronously
writeback any dirty mmap(2) regions prior to reporting holes.

Additionally, while not strictly required, the dn_struct_rwlock is
now held over the dirty check to prevent the dnode structure from
changing.  This ensures that a clean dnode can't be dirtied before
the data/hole is located.  The range lock is now also taken to
ensure the call cannot race with zfs_write().

Furthermore, the code was refactored to provide a dnode_is_dirty()
helper function which checks the dnode for any dirty records to
determine its dirtiness.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Rich Ercolani <rincebrain@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #11900
Closes #12724

include/os/linux/zfs/sys/zfs_znode_impl.h

@@ -71,6 +71,7 @@ extern "C" {
 #define	Z_ISDIR(type)	S_ISDIR(type)
 
 #define	zn_has_cached_data(zp)		((zp)->z_is_mapped)
+#define	zn_flush_cached_data(zp, sync)	write_inode_now(ZTOI(zp), sync)
 #define	zn_rlimit_fsize(zp, uio)	(0)
 
 /*