mtd: sm_ftl: heap corruption in sm_create_sysfs_attributes()

We always put a NUL terminator one space past the end of the "vendor"
buffer.  Walter Harms also pointed out that this should just use
kstrndup().

Fixes: 7d17c02a ('mtd: Add new SmartMedia/xD FTL')

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>