fs/proc/generic.c: fix incorrect pde_is_permanent check

Currently the pde_is_permanent() check is being run on root multiple times
rather than on the next proc directory entry.  This looks like a
copy-paste error.  Fix this by replacing root with next.

Addresses-Coverity: ("Copy-paste error")
Link: https://lkml.kernel.org/r/20210318122633.14222-1-colin.king@canonical.com
Fixes: d919b33d

 ("proc: faster open/read/close with "permanent" files")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>