btrfs: move the block group freeze/unfreeze helpers into block-group.c

The helpers btrfs_freeze_block_group() and btrfs_unfreeze_block_group()
used to be named btrfs_get_block_group_trimming() and
btrfs_put_block_group_trimming() respectively.

At the time they were added to free-space-cache.c, by commit e33e17ee
("btrfs: add missing discards when unpinning extents with -o discard")
because all the trimming related functions were in free-space-cache.c.

Now that the helpers were renamed and are used in scrub context as well,
move them to block-group.c, a much more logical location for them.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>