client: preserve CEPH_SETATTR_ATIME mask when timestamps match

Fix an optimization trap where the client strips CEPH_SETATTR_ATIME
from the setattr mask if the requested atime matches the locally
cached inode atime. The fix ensures that if the client does not
hold EXCL or WR caps, the CEPH_SETATTR_ATIME bit is preserved
in the network mask. This forces the MDS to process the transaction,
allowing it to execute the mandatory ctime advancement.

Fixes: https://tracker.ceph.com/issues/71639
Signed-off-by: Jos Collin <jcollin@redhat.com>