lightnvm: remove function name in strings

For the sysfs functions, the function names are embedded into their
error strings. If the function name later changes, the string may
not be updated accordingly. Update the strings to use __func__
to avoid this.

Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>