Fix some coding style errors in memblock.c

This patch removes the initialization of some static variables to 0 and
`false` in the memblock source file, according to the coding style
guidelines.

Signed-off-by: Claudio Migliorelli <claudio.migliorelli@mail.polimi.it>
Link: https://lore.kernel.org/r/87r0sa7mm8.fsf@mail.polimi.it
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>