fat: avoid stack overflow warning

fat: avoid stack overflow warning

Building the fat kunit tests on with -fsanitize=alignment reveals some
rather excessive stack usage:

fs/fat/fat_test.c: In function 'fat_clus_to_blknr_test':
fs/fat/fat_test.c:33:1: error: the frame size of 4736 bytes is larger than 1536 bytes [-Werror=frame-larger-than=]
   33 | }
      | ^
fs/fat/fat_test.c: In function 'fat_get_blknr_offset_test':
fs/fat/fat_test.c:52:1: error: the frame size of 4800 bytes is larger than 1536 bytes [-Werror=frame-larger-than=]

The problem is clearly related to the on-stack copy of a local
msdos_sb_info structure.  Avoid this by making that copy 'static const'
and changing the called functions to accept a constant input.

Link: https://lore.kernel.org/20260515204456.2692208-1-arnd@kernel.org
Fixes: 410002f8139c ("kunit: fat: test cluster and directory i_pos layout helpers")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Adi Nata <adinata.softwareengineer@gmail.com>
Cc: David Laight <david.laight.linux@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>