g_file_set_contents(): don't fsync on ext3/4

g_file_set_contents(): don't fsync on ext3/4

ext3 and ext4 (for quite some time) with default mount options don't
need fsync() to ensure safety of replace-by-rename.  Stop doing that for
these filesystems.

Note: this patch also impacts ext2, which is probably not safe, but I
don't know of any way to check ext2. vs the others because they all have
the same magic numbers (short of opening /proc/mount).

This patch assumes that if BTRFS_SUPER_MAGIC is defined then so will be
EXT3_SUPER_MAGIC.

https://bugzilla.gnome.org/show_bug.cgi?id=701560