btrfs: avoid unnecessary memory allocation and copy at overwrite_item()

There's no need to allocate memory and copy from both the destination and
source extent buffers to compare if the items are equal, we can instead
use memcmp_extent_buffer() which allows to do only one memory allocation
and copy instead of two.

So use memcmp_extent_buffer() instead of memcmp(), allowing us to avoid
one memory allocation, which can fail or be slow while under memory heavy
pressure, avoid the memory copying and reducing code.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>