btrfs: reduce size and overhead of extent_map_block_end()

btrfs: reduce size and overhead of extent_map_block_end()

At extent_map_block_end() we are calling the inline functions
extent_map_block_start() and extent_map_block_len() multiple times, which
results in expanding their code multiple times, increasing the compiled
code size and repeating the computations those functions do.

Improve this by caching their results in local variables.

The size of the module before this change:

   $ size fs/btrfs/btrfs.ko
      text	   data	    bss	    dec	    hex	filename
   1755770	 163800	  16920	1936490	 1d8c6a	fs/btrfs/btrfs.ko

And after this change:

   $ size fs/btrfs/btrfs.ko
      text	   data	    bss	    dec	    hex	filename
   1755656	 163800	  16920	1936376	 1d8bf8	fs/btrfs/btrfs.ko

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
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>