openrisc: use generic strncpy_from_user

As per commits 2922585b ("lib: Sparc's strncpy_from_user is generic
enough, move under lib/") and 92ae03f2 ("x86: merge 32/64-bit
versions of 'strncpy_from_user()' and speed it up"), and corresponding
discussion on linux-arch.

Signed-off-by: Jonas Bonn <jonas@southpole.se>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>