mm: shmem: use signed int for version handling in casefold option

Fixes an issue where the use of an unsigned data type in
`shmem_parse_opt_casefold()` caused incorrect evaluation of negative
conditions.

Link: https://lkml.kernel.org/r/20250111-unsignedcompare1601569-v3-1-c861b4221831@gmail.com


Fixes: 58e55efd ("tmpfs: Add casefold lookup support")
Reviewed-by: André Almeida <andrealmeid@igalia.com>
Reviewed-by: Gabriel Krisman Bertazi <gabriel@krisman.be>
Signed-off-by: Karan Sanghavi <karansanghvi98@gmail.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Hugh Dickens <hughd@google.com>
Cc: Shuah khan <skhan@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>