dma-buf/dma-resv: Respect num_fences when initializing the shared fence list.

We hardcode the maximum number of shared fences to 4, instead of
respecting num_fences. Use a minimum of 4, but more if num_fences
is higher.

This seems to have been an oversight when first implementing the
api.

Fixes: 04a5faa8

 ("reservation: update api and add some helpers")
Cc: <stable@vger.kernel.org> # v3.17+
Reported-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201124115707.406917-1-maarten.lankhorst@linux.intel.com