ethtool: Allow zero-length register dumps again

ethtool: Allow zero-length register dumps again

Some drivers (ab)use the ethtool_ops::get_regs operation to expose
only a hardware revision ID.  Commit
a77f5db361ed9953b5b749353ea2c7fed2bf8d93 ('ethtool: Allocate register
dump buffer with vmalloc()') had the side-effect of breaking these, as
vmalloc() returns a null pointer for size=0 whereas kmalloc() did not.

For backward-compatibility, allow zero-length dumps again.

Reported-by: Kalle Valo <[email protected]>
Signed-off-by: Ben Hutchings <[email protected]>
Cc: [email protected] [2.6.37+]
Signed-off-by: David S. Miller <[email protected]>