debugfs: be more robust at handling improper input in debugfs_lookup()

debugfs_lookup() doesn't like it if it is passed an illegal name
pointer, or if the filesystem isn't even initialized yet.  If either of
these happen, it will crash the system, so fix it up by properly testing
for valid input and that we are up and running before trying to find a
file in the filesystem.

Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: stable <stable@vger.kernel.org>
Reported-by: Michael Walle <michael@walle.cc>
Tested-by: Michael Walle <michael@walle.cc>
Tested-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210218100818.3622317-1-gregkh@linuxfoundation.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>