proc: avoid extra pde_put() in proc_fill_super()

If proc_get_inode() succeeded, but d_make_root() failed, pde_put() for
proc_root will be called twice: the first time due to iput() called from
d_make_root() and the second time directly in the end of proc_fill_super().

Signed-off-by: Maxim Patlasov <mpatlasov@parallels.com>