IB/mad: Use krealloc() to resize snoop table

Use krealloc() instead of kmalloc() followed by memcpy() when resizing
the MAD module's snoop table.

Also put parentheses around the new table size to avoid calculating
the wrong size to allocate, which fixes a bug pointed out by Haven
Hash <haven.hash@isilon.com>.

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