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.