IPoIB: Allocate priv->tx_ring with vmalloc()

Commit 7143740d

 ("IPoIB: Add send gather support") made struct
ipoib_tx_buf significantly larger, since the mapping member changed
from a single u64 to an array with MAX_SKB_FRAGS + 1 entries.  This
means that allocating tx_rings with kzalloc() may fail because there
is not enough contiguous memory for the new, much bigger size.  Fix
this regression by allocating the rings with vmalloc() instead.

Signed-off-by: Roland Dreier <rolandd@cisco.com>