btrfs: add unlikely to critical error in btrfs_extend_item()

btrfs: add unlikely to critical error in btrfs_extend_item()

It's not expected to get a data size less than the leaf's free space,
which would lead to a leaf dump and BUG(), so tag the if statement's
expression as unlikely, hinting the compiler to potentially generate
better code.

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