r_safe_completion is currently, and has always been, signaled only if
on-disk ack was requested.  It's there for fsync and syncfs, which wait
for in-flight writes to flush - all data write requests set ONDISK.

However, the pool perm check code introduced in 4.2 sends a write
request with only ACK set.  An unfortunately timed syncfs can then hang
forever: r_safe_completion won't be signaled because only an unsafe
reply was requested.

We could patch ceph_osdc_sync() to skip !ONDISK write requests, but
that is somewhat incomplete and yet another special case.  Instead,
rename this completion to r_done_completion and always signal it when
the OSD client is done with the request, whether unsafe, safe, or
error.  This is a bit cleaner and helps with the cancellation code.

Reported-by: Yan, Zheng <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>