btrfs: rename free_extent_state() to include a btrfs prefix

btrfs: rename free_extent_state() to include a btrfs prefix

This is an exported function so it should have a 'btrfs_' prefix by
convention, to make it clear it's btrfs specific and to avoid collisions
with functions from elsewhere in the kernel.

Rename the function to add 'btrfs_' prefix to it.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>