fs: namespace: Avoid -Wflex-array-member-not-at-end warning

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Move the conflicting declaration to the end of the structure. Notice
that `struct statmount` is a flexible structure --a structure that
contains a flexible-array member.

Fix the following warning:

fs/namespace.c:5329:26: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/Z-SZKNdCiAkVJvqm@kspp


Signed-off-by: Christian Brauner <brauner@kernel.org>