kernfs: combine ino/id lookup functions into kernfs_find_and_get_node_by_id()

kernfs_find_and_get_node_by_ino() looks the kernfs_node matching the
specified ino.  On top of that, kernfs_get_node_by_id() and
kernfs_fh_get_inode() implement full ID matching by testing the rest
of ID.

On surface, confusingly, the two are slightly different in that the
latter uses 0 gen as wildcard while the former doesn't - does it mean
that the latter can't uniquely identify inodes w/ 0 gen?  In practice,
this is a distinction without a difference because generation number
starts at 1.  There are no actual IDs with 0 gen, so it can always
safely used as wildcard.

Let's simplify the code by renaming kernfs_find_and_get_node_by_ino()
to kernfs_find_and_get_node_by_id(), moving all lookup logics into it,
and removing now unnecessary kernfs_get_node_by_id().

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/kernfs.h

@@ -393,7 +393,8 @@ void kernfs_kill_sb(struct super_block *sb);
 
 void kernfs_init(void);
 
-struct kernfs_node *kernfs_get_node_by_id(struct kernfs_root *root, u64 id);
+struct kernfs_node *kernfs_find_and_get_node_by_id(struct kernfs_root *root,
+						   u64 id);
 #else	/* CONFIG_KERNFS */
 
 static inline enum kernfs_node_type kernfs_type(struct kernfs_node *kn)