btrfs: open code btrfs_map_sblock

btrfs_map_sblock just hard codes three arguments and calls
btrfs_map_sblock.  Remove it as it doesn't provide any real value, but
makes following the btrfs_map_block call chains harder.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>