btrfs: consolidate the error handling in run_delalloc_nocow

btrfs: consolidate the error handling in run_delalloc_nocow

Share the calls to extent_clear_unlock_delalloc for btrfs_path allocation
failure handling and the normal exit path.

This relies on btrfs_free_path ignoring a NULL pointer, and the
initialization of cur_offset to start at the beginning of the function.

Reviewed-by: Boris Burkov <[email protected]>
Signed-off-by: Christoph Hellwig <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>