hwrng: core - Fix page fault dead lock on mmap-ed hwrng

In the Linux kernel, the following vulnerability has been resolved: hwrng: core - Fix page fault dead lock on mmap-ed hwrng There is a dead-lock in the hwrng device read path. This triggers when the user reads from /dev/hwrng into memory also mmap-ed from /dev/hwrng. The resulting page fault triggers a recursive read which then dead-locks. Fix this by using a stack buffer when calling copy_to_user.