gfs2: Support negative atimes

When inodes are read from disk, GFS2 will only update in-memory atimes
older than the on-disk atimes; this prevents atimes from going
backwards.  The atimes of newly allocated inodes are initialized to 0.
This means that when an atime is explicitly set to a negative value,
this value will not persist.

Fix by setting the atime of newly allocated inodes to the lowest
possible value instead of 0.

Fixes xfstest generic/258.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>