md: replace seq_release_private with seq_release

Since commit f1514638

 ("fs: seq_file - add event counter to simplify
poll() support"), md.c code has been no longer used the private field of
the struct seq_file, but seq_release_private() has been continued to be
used to release the allocated seq_file. This seems to have been
forgotten. So convert it to use seq_release() instead of
seq_release_private().

Signed-off-by: Cihangir Akturk <cakturk@gmail.com>
Signed-off-by: Shaohua Li <shli@fb.com>