btrfs: check BLOCK_GROUP_FLAG_NEEDS_FREE_SPACE at __add_block_group_free_space()

btrfs: check BLOCK_GROUP_FLAG_NEEDS_FREE_SPACE at __add_block_group_free_space()

Every caller of __add_block_group_free_space() is checking if the flag
BLOCK_GROUP_FLAG_NEEDS_FREE_SPACE is set before calling it. This is
duplicate code and it's prone to some mistake in case we add more callers
in the future. So move the check for that flag into the start of
__add_block_group_free_space(), and, as a consequence, the path allocation
from add_block_group_free_space() is moved into
__add_block_group_free_space(), to preserve the behaviour of allocating
a path only if the flag BLOCK_GROUP_FLAG_NEEDS_FREE_SPACE is set.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>