libfdt: Fix handling of trailing / in fdt_path_offset()

Currently, fdt_path_offset() returns FDL_ERR_BADOFFSET if given a path
with a trailing '/'.  In particular this means that
fdt_path_offset("/") returns FDT_ERR_BADOFFSET rather than 0 as one
would expect.

This patch fixes the function to accept and ignore trailing '/'
characters.  As well as allowing fdt_path_offset("/") this means that
fdt_path_offset("/foo/") will return the same as
fdt_path_offset("/foo") which seems in keeping with the principle of
least surprise.

This also adds a testcase to ensure that fdt_path_offset("/") returns
0 as it should.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

libfdt/fdt_ro.c

@@ -154,7 +154,7 @@ int fdt_path_offset(const void *fdt, const char *path)
 		while (*p == '/')
 			p++;
 		if (! *p)
-			return -FDT_ERR_BADPATH;
+			return offset;
 		q = strchr(p, '/');
 		if (! q)
 			q = end;

tests/path_offset.c

@@ -58,6 +58,7 @@ int check_subnode(void *fdt, int parent, const char *name)
 int main(int argc, char *argv[])
 {
 	void *fdt;
+	int root_offset;
 	int subnode1_offset, subnode2_offset;
 	int subnode1_offset_p, subnode2_offset_p;
 	int subsubnode1_offset, subsubnode2_offset;
@@ -66,6 +67,13 @@ int main(int argc, char *argv[])
 	test_init(argc, argv);
 	fdt = load_blob_arg(argc, argv);
 
+	root_offset = fdt_path_offset(fdt, "/");
+	if (root_offset < 0)
+		FAIL("fdt_path_offset(\"/\") failed: %s",
+		     fdt_strerror(root_offset));
+	else if (root_offset != 0)
+		FAIL("fdt_path_offset(\"/\") returns incorrect offset %d",
+		     root_offset);
 	subnode1_offset = check_subnode(fdt, 0, "subnode1");
 	subnode2_offset = check_subnode(fdt, 0, "subnode2");