Staging: exfat: Avoid use of strcpy

Use strscpy instead of strcpy in exfat_core.c, and add a check
for length that will return already known FFS_INVALIDPATH.

Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Sandro Volery <sandro@volery.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20190912082559.GA5043@volery


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