Fix false ENOENT on snapshot control dentries

Lookups in the snapshot control directory for an existing snapshot
fail with ENOENT if an earlier lookup failed before the snapshot was
created.  This is because the earlier lookup causes a negative dentry
to be cached which is never invalidated.

The bug can be reproduced as follows (the second ls should succeed):

 $ ls /tank/.zfs/snapshot/s
 ls: cannot access /tank/.zfs/snapshot/s: No such file or directory
 $ zfs snap tank@s
 $ ls /tank/.zfs/snapshot/s
 ls: cannot access /tank/.zfs/snapshot/s: No such file or directory

To remedy this, always invalidate cached dentries in the snapshot
control directory.  Since these entries never exist on disk there is
no significant performance penalty for the extra lookups.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1192

include/linux/dcache_compat.h

@@ -35,4 +35,29 @@
 #define d_make_root(inode)	d_alloc_root(inode)
 #endif /* HAVE_D_MAKE_ROOT */
 
+/*
+ * 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.
+ */
+#ifdef HAVE_CONST_DENTRY_OPERATIONS
+typedef const struct dentry_operations	dentry_operations_t;
+#else
+typedef struct dentry_operations	dentry_operations_t;
+#endif
+
+/*
+ * 2.6.38 API change,
+ * Added d_set_d_op() helper function which sets some flags in
+ * dentry->d_flags based on which operations are defined.
+ */
+#ifndef HAVE_D_SET_D_OP
+static inline void
+d_set_d_op(struct dentry *dentry, dentry_operations_t *op)
+{
+	dentry->d_op = op;
+}
+#endif /* HAVE_D_SET_D_OP */
+
 #endif /* _ZFS_DCACHE_H */