btrfs: drop redundant bflags initialization when allocating extent buffer

When allocating an extent buffer, at __alloc_extent_buffer(), there's no
point in explicitly assigning zero to the bflags field of the new extent
buffer because we allocated it with kmem_cache_zalloc().

So just remove the redundant initialization, it saves one mov instruction
in the generated assembly code for x86_64 ("movq $0x0,0x10(%rax)").

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