ext4: fix up rb_root initializations to use RB_ROOT
ext4 uses rb_node = NULL; to zero rb_root at few places. Using
RB_ROOT as the initializer is more portable in case the underlying
implementation of rbtrees changes in the future.
Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: "Theodore Ts'o" <tytso@mit.edu> Cc: Eric Paris <eparis@redhat.com>