tlsf_malloc: dummy_malloc: Share code for calloc() ...

Calloc() implementation for TLSF does not correctly check for malloc()
failure which can result in a NULL pointer exception when trying to
calloc() extra large buffers.

Since both TLSF and dummy malloc implementations of calloc() are
exactly the same, pick implementation for the latter (which does
aforementioned check) and share it between the two.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>