fs: dcache reduce branches in lookup path

Reduce some branches and memory accesses in dcache lookup by adding dentry
flags to indicate common d_ops are set, rather than having to check them.
This saves a pointer memory access (dentry->d_op) in common path lookup
situations, and saves another pointer load and branch in cases where we
have d_op but not the particular operation.

Patched with:

git grep -E '[.>]([[:space:]])*d_op([[:space:]])*=' | xargs sed -e 's/\([^\t ]*\)->d_op = \(.*\);/d_set_d_op(\1, \2);/' -e 's/\([^\t ]*\)\.d_op = \(.*\);/d_set_d_op(\&\1, \2);/' -i

Signed-off-by: Nick Piggin <npiggin@kernel.dk>
diff --git a/fs/pipe.c b/fs/pipe.c
index ae3592d..01a7865 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -1004,7 +1004,7 @@
 		goto err_inode;
 	path.mnt = mntget(pipe_mnt);
 
-	path.dentry->d_op = &pipefs_dentry_operations;
+	d_set_d_op(path.dentry, &pipefs_dentry_operations);
 	d_instantiate(path.dentry, inode);
 
 	err = -ENFILE;