fs: document and rename fsid helpers

Vivek pointed out that the fs{g,u}id_into_mnt() naming scheme can be
misleading as it could be understood as implying they do the exact same
thing as i_{g,u}id_into_mnt(). The original motivation for this naming
scheme was to signal to callers that the helpers will always take care
to map the k{g,u}id such that the ownership is expressed in terms of the
mnt_users.
Get rid of the confusion by renaming those helpers to something more
sensible. Al suggested mapped_fs{g,u}id() which seems a really good fit.
Usually filesystems don't need to bother with these helpers directly
only in some cases where they allocate objects that carry {g,u}ids which
are either filesystem specific (e.g. xfs quota objects) or don't have a
clean set of helpers as inodes have.

Link: https://lore.kernel.org/r/20210320122623.599086-3-christian.brauner@ubuntu.com
Inspired-by: Vivek Goyal <vgoyal@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Darrick J. Wong <djwong@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

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