sysfs: clean up sysfs_get_dirent()

The pre-existing sysfs interfaces which take explicit namespace
argument are weird in that they place the optional @ns in front of
@name which is contrary to the established convention.  For example,
we end up forcing vast majority of sysfs_get_dirent() users to do
sysfs_get_dirent(parent, NULL, name), which is silly and error-prone
especially as @ns and @name may be interchanged without causing
compilation warning.

This renames sysfs_get_dirent() to sysfs_get_dirent_ns() and swap the
positions of @name and @ns, and sysfs_get_dirent() is now a wrapper
around sysfs_get_dirent_ns().  This makes confusions a lot less
likely.

There are other interfaces which take @ns before @name.  They'll be
updated by following patches.

This patch doesn't introduce any functional changes.

v2: EXPORT_SYMBOL_GPL() wasn't updated leading to undefined symbol
    error on module builds.  Reported by build test robot.  Fixed.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Kay Sievers <kay@vrfy.org>
Cc: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
index e784340..0f3214a 100644
--- a/fs/sysfs/file.c
+++ b/fs/sysfs/file.c
@@ -563,7 +563,7 @@
 	int error;
 
 	if (group)
-		dir_sd = sysfs_get_dirent(kobj->sd, NULL, group);
+		dir_sd = sysfs_get_dirent(kobj->sd, group);
 	else
 		dir_sd = sysfs_get(kobj->sd);
 
@@ -645,7 +645,7 @@
 	struct sysfs_dirent *dir_sd;
 
 	if (group)
-		dir_sd = sysfs_get_dirent(kobj->sd, NULL, group);
+		dir_sd = sysfs_get_dirent(kobj->sd, group);
 	else
 		dir_sd = sysfs_get(kobj->sd);
 	if (dir_sd) {