btrfs: use kmalloc_array() for open-coded arithmetic in kmalloc()

btrfs: use kmalloc_array() for open-coded arithmetic in kmalloc()

As pointed out in the documentation, calling 'kmalloc' with open-coded
arithmetic can lead to unfortunate overflows and this particular way of
using it has been deprecated. Instead, it's preferred to use
'kmalloc_array' in cases where it might apply so an overflow check is
performed.

Note this is an API cleanup and is not fixing any overflows because in
all cases the multipliers are bounded small numbers derived from number
of items in leaves/nodes.

Signed-off-by: Miquel Sabaté Solà <mssola@mssola.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>