fs: add ctime accessors infrastructure

[ Upstream commit 9b6304c1

 ]

struct timespec64 has unused bits in the tv_nsec field that can be used
for other purposes. In future patches, we're going to change how the
inode->i_ctime is accessed in certain inodes in order to make use of
them. In order to do that safely though, we'll need to eradicate raw
accesses of the inode->i_ctime field from the kernel.

Add new accessor functions for the ctime that we use to replace them.

Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Message-Id: <20230705185812.579118-2-jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Stable-dep-of: 5923d668

 ("smb3: fix caching of ctime on setxattr")
Signed-off-by: Sasha Levin <sashal@kernel.org>