btrfs: make set_extent_bit helpers static inline

The funcions just wrap the set_extent_bit API and generate function
calls. This increases stack consumption and may negatively affect
performance due to icache misses. We can simply make the helpers static
inline and keep the type checking and API untouched. The code slightly
increases:

   text	   data	    bss	    dec	    hex	filename
 938427	  43670	  23144	1005241	  f56b9	fs/btrfs/btrfs.ko.before
 938667	  43670	  23144	1005481	  f57a9	fs/btrfs/btrfs.ko

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