docs: admin-guide: sysctl: fix details of struct dentry_stat_t

Commit c8c0c239 moved struct dentry_stat_t to fs/dcache.c but
did not update its location in Documentation, so update that now.
Also change each struct member from int to long as done in
commit 3942c07c.

Fixes: c8c0c239 ("fs: move dcache sysctls to its own file")
Fixes: 3942c07c ("fs: bump inode and dentry counters to long")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: linux-fsdevel@vger.kernel.org
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Glauber Costa <glommer@openvz.org>
Acked-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230923195144.26043-1-rdunlap@infradead.org