btrfs: extent_buffer_uptodate() make it static and inline

extent_buffer_uptodate() is a trivial wrapper around test_bit() and
nothing else. So make it static and inline, save on code space and call
indirection.

Before:
   text	   data	    bss	    dec	    hex	filename
1131257	  82898	  18992	1233147	 12d0fb	fs/btrfs/btrfs.ko

After:
   text	   data	    bss	    dec	    hex	filename
1131090	  82898	  18992	1232980	 12d054	fs/btrfs/btrfs.ko

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>