efivarfs: Don't return -EINTR when rate-limiting reads

efivarfs: Don't return -EINTR when rate-limiting reads

Applications that read EFI variables may see a return
value of -EINTR if they exceed the rate limit and a
signal delivery is attempted while the process is sleeping.

This is quite surprising to the application, which probably
doesn't have code to handle it.

Change the interruptible sleep to a non-interruptible one.

Reported-by: Lennart Poettering <mzxreary@0pointer.de>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Link: https://lore.kernel.org/r/20200528194905.690-3-tony.luck@intel.com
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>