btrfs: rename __get_extent_map() and pass btrfs_inode

btrfs: rename __get_extent_map() and pass btrfs_inode

[ Upstream commit 06de96faf795b5c276a3be612da6b08c6112e747 ]

The double underscore naming scheme does not apply here, there's only
only get_extent_map(). As the definition is changed also pass the struct
btrfs_inode.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Stable-dep-of: acc18e1c1d8c ("btrfs: fix stale page cache after race between readahead and direct IO write")
Signed-off-by: Sasha Levin <sashal@kernel.org>