LU-10499 pcc: print help msg more clearly for detach

LU-10499 pcc: print help msg more clearly for detach

In this patch, it prints the help message for detach_fid and
detach commands more clearly when not given the required
parameters such as mount point or FIDs.

It also ignores the -EINPROGRESS error if the file is being
attached, i.e. copy data from Lustre OSTs into PCC.

Reset llite.*.pcc_async_threshold to the original value when the
subtest exits, so that it doesn't affect later tests.

Wait for async pcc attach to finish in test_47.

EX-bug-id: EX-3814 EX-3409
Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I547e80e5c9c213b159039b9b79da176cdb91c4bc
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54425
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>