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At find_extent_clone() we are initializing to zero the 'found_itself' and
'found' fields of the backref context before we use it but we have already
initialized the structure to zeroes when we declared it on stack, so it's
pointless to initialize those fields and they are unnecessarily increasing
the object text size with two "mov" instructions (x86_64).

Similarly make the 'extent_len' initialization more clear by using an if-
-then-else instead of a double assignment to it in case the extent's end
crosses the i_size boundary.

Before this change:

   $ size fs/btrfs/send.o
      text	   data	    bss	    dec	    hex	filename
     68694	   4252	     16	  72962	  11d02	fs/btrfs/send.o

After this change:

   $ size fs/btrfs/send.o
      text	   data	    bss	    dec	    hex	filename
     68678	   4252	     16	  72946	  11cf2	fs/btrfs/send.o
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>