debugfs: fix set_field's handling of timestamps

debugfs: fix set_field's handling of timestamps

How timestamps are encoded in inodes and superblocks are different.
Unfortunately, commit ca8bc9240a00 which added post-2038 timestamps
was (a) overwriting adjacent superblock fields and/or attempting
unaligned writes to a 8-bit field from a 32-bit pointer, and (b) using
the incorrect encoding for timestamps stored in inodes.  Fix both of
these issues, which were found thanks to UBSAN.

Fixes: ca8bc9240a00 ("Add post-2038 timestamp support to e2fsprogs")
Signed-off-by: Theodore Ts'o <tytso@mit.edu>