Fix coverity false positives introduced by the post-2038 changes

Fix coverity false positives introduced by the post-2038 changes

Commit ca8bc9240a00 ("Add post-2038 timestamp support...") did things
like casting a 64-bit unsigned integer into a signed 32-bit integer
deliberately; but Coverity thinks this is a bug.  So mask off the bits
to make it clear this was deliberate.

Addresses-Coverity-Bug: 1596519
Addresses-Coverity-Bug: 1596515
Addresses-Coverity-Bug: 1596514
Addresses-Coverity-Bug: 1596513
Addresses-Coverity-Bug: 1596511
Addresses-Coverity-Bug: 1596509
Addresses-Coverity-Bug: 1596508
Addresses-Coverity-Bug: 1596504
Addresses-Coverity-Bug: 1596502
Addresses-Coverity-Bug: 1596501
Signed-off-by: Theodore Ts'o <tytso@mit.edu>