libceph: nuke ceph_kvfree()

Use kvfree() from linux/mm.h instead, which is identical.  Also fix the
ceph_buffer comment: we will allocate with kmalloc() up to 32k - the
value of PAGE_ALLOC_COSTLY_ORDER, but that really is just an
implementation detail so don't mention it at all.

Signed-off-by: Ilya Dryomov <idryomov@redhat.com>

include/linux/ceph/buffer.h

@@ -10,8 +10,7 @@
 /*
  * a simple reference counted buffer.
  *
- * use kmalloc for small sizes (<= one page), vmalloc for larger
- * sizes.
+ * use kmalloc for smaller sizes, vmalloc for larger sizes.
  */
 struct ceph_buffer {
 	struct kref kref;