sheep: use 1M log space for non-debug mode startup

This will save a lot of memory footprint of sheep daemon

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
sheep: fix default_link on EEXIST of link

This is observed by the following bug:

Jun 18 21:55:11 [rw] default_link(377) failed to link from
/mnt/ST2000DM001-1CH164_W1E2N5GM/obj/.stale/0002e519000054f1.21 to
/mnt/ST2000DM001-1CH164_W1E2N5GM/obj/0002e519000054f1, File exists
Jun 18 21:55:11 [rw] err_to_sderr(96) oid=2e519000054f1, File exists
Jun 18 21:55:11 [main] remove_disk(319) /mnt/ST2000DM001-1CH164_W1E2N5GM/obj from multi-disk array

Reported-by: Valerio Pachera <sirio81@gmail.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
util: consolidate atomic_create_and_write to be concurrency friendly

Only one thread can call this function on the specified path.

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>