btrfs: open code timespec64 in struct btrfs_inode

[ Upstream commit c6e8f898 ]

The type of timespec64::tv_nsec is 'unsigned long', while we have only
u32 for on-disk and in-memory. This wastes a few bytes in btrfs_inode.
Add separate members for sec and nsec with the corresponding type width.
This creates a 4 byte hole in btrfs_inode which can be utilized in the
future.

Signed-off-by: David Sterba <dsterba@suse.com>
Stable-dep-of: 1ef94169 ("btrfs: populate otime when logging an inode item")
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>