vxlan: check return value of gro_cells_init()

gro_cells_init() returns error if memory allocation is failed.
But the vxlan module doesn't check the return value of gro_cells_init().

Fixes: 58ce31cc ("vxlan: GRO support at tunnel layer")`
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>