ext4: ext4_ext_migrate should use NULL not 0

ext4_ext_migrate() calls ext4_new_inode() and passes 0 instead of a pointer
to a struct qstr.  This patch uses NULL, to make it obvious to the caller
that this was a pointer.

Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>