libcxgbi: use kvfree() in cxgbi_free_big_mem()

Use kvfree() instead of open-coding it.

Signed-off-by: Pekka Enberg <penberg@kernel.org>
Cc: "James E.J. Bottomley" <JBottomley@odin.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>