btrfs: send: add unlikely to all unexpected overflow checks

btrfs: send: add unlikely to all unexpected overflow checks

There are several checks for unexpected overflows of buffers and path
lengths that makes us fail the send operation with an error if for some
highly unexpected reason they happen. So add the unlikely tag to those
checks to hint the compiler to generate better code, while also making
it more explicit in the source that it's highly unexpected.

With gcc 14.2.0-19 from Debian on x86_64, I also got a small reduction
the text size of the btrfs module.

Before:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  1936917	 162723	  15592	2115232	 2046a0	fs/btrfs/btrfs.ko

After:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  1936789	 162723	  15592	2115104	 204620	fs/btrfs/btrfs.ko

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>