fuse2fs: fix relatime comparisons

fuse2fs: fix relatime comparisons

generic/192 fails like this even before we start adding iomap code:

    --- tests/generic/192.out   2025-04-30 16:20:44.512675591 -0700
    +++ tests/generic/192.out.bad    2025-07-06 22:26:11.666015735 -0700
    @@ -1,5 +1,6 @@
     QA output created by 192
     sleep for 5 seconds
     test
    -delta1 is in range
    +delta1 has value of 0
    +delta1 is NOT in range 5 .. 7
     delta2 is in range

The cause of this regression is that the timestamp comparisons account
only for seconds, not nanoseconds.  If a write came in 100ms after the
last read but still in the same second, then we fail to update atime on
a subsequent read.

Fix this by converting the timespecs to doubles so that we can include
the nanoseconds component, and then perform the comparison in floating
point mode.

Cc: linux-ext4@vger.kernel.org # v1.43
Fixes: 81cbf1ef4f5dab ("misc: add fuse2fs, a FUSE server for e2fsprogs")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Link: https://lore.kernel.org/r/20250707160525.GC2672022@frogsfrogsfrogs
Signed-off-by: Theodore Ts'o <tytso@mit.edu>