Linux 6.17: d_set_d_op() is no longer available

We only have extremely narrow uses, so move it all into a single
function that does only what we need, with and without d_set_d_op().

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #17621

include/os/linux/kernel/linux/dcache_compat.h

@@ -60,32 +60,6 @@
 	} while (0)
 #endif
 
-/*
- * 2.6.30 API change,
- * The const keyword was added to the 'struct dentry_operations' in
- * the dentry structure.  To handle this we define an appropriate
- * dentry_operations_t typedef which can be used.
- */
-typedef const struct dentry_operations	dentry_operations_t;
-
-/*
- * 2.6.38 API addition,
- * Added d_clear_d_op() helper function which clears some flags and the
- * registered dentry->d_op table.  This is required because d_set_d_op()
- * issues a warning when the dentry operations table is already set.
- * For the .zfs control directory to work properly we must be able to
- * override the default operations table and register custom .d_automount
- * and .d_revalidate callbacks.
- */
-static inline void
-d_clear_d_op(struct dentry *dentry)
-{
-	dentry->d_op = NULL;
-	dentry->d_flags &= ~(
-	    DCACHE_OP_HASH | DCACHE_OP_COMPARE |
-	    DCACHE_OP_REVALIDATE | DCACHE_OP_DELETE);
-}
-
 /*
  * Walk and invalidate all dentry aliases of an inode
  * unless it's a mountpoint