Make ->d_sb assign-once and always non-NULL

New helper (non-exported, fs/internal.h-only): __d_alloc(sb, name).
Allocates dentry, sets its ->d_sb to given superblock and sets
->d_op accordingly.  Old d_alloc(NULL, name) callers are converted
to that (all of them know what superblock they want).  d_alloc()
itself is left only for parent != NULl case; uses __d_alloc(),
inserts result into the list of parent's children.

Note that now ->d_sb is assign-once and never NULL *and*
->d_parent is never NULL either.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/internal.h

@@ -135,3 +135,8 @@ extern void inode_wb_list_del(struct inode *inode);
 extern int get_nr_dirty_inodes(void);
 extern void evict_inodes(struct super_block *);
 extern int invalidate_inodes(struct super_block *, bool);
+
+/*
+ * dcache.c
+ */
+extern struct dentry *__d_alloc(struct super_block *, const struct qstr *);