libceph: switch osdmap_set_max_osd() to krealloc()

Use krealloc() instead of rolling our own.  (krealloc() with a NULL
first argument acts as a kmalloc()).  Properly initalize the new array
elements.  This is needed to make future additions to osdmap easier.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Alex Elder <elder@linaro.org>