powerpc/pseries: skip reserved LMBs in dlpar_memory_add_by_count()

The function is counting reserved LMBs as available to be added, but
they aren't. This will cause the function to miscalculate the available
LMBs and can trigger errors later on when executing dlpar_add_lmb().

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210622133923.295373-2-danielhb413@gmail.com