btrfs: unexport btrfs_qgroup_reserve_meta()

btrfs: unexport btrfs_qgroup_reserve_meta()

There's only one caller outside qgroup.c of btrfs_qgroup_reserve_meta()
and we have btrfs_qgroup_reserve_meta_prealloc() is a wrapper around
that function. Make that caller use btrfs_qgroup_reserve_meta_prealloc()
and unexport btrfs_qgroup_reserve_meta(), simplifying the external API.

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>