staging: rts_pstor: use kbasename()

staging: rts_pstor: use kbasename()

The custom filename function mostly repeats the kernel's kbasename. This patch
simplifies it. The updated filename() will not check for the '\' in the
filenames. It seems redundant in Linux. The __FILE__ macro always defined if
we compile an existing file. Thus, NULL check is not needed there as well.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: YAMANE Toshiaki <yamanetoshi@gmail.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>