Linux: bump -std to gnu11

Linux switched from -std=gnu89 to -std=gnu11 in 5.18
(torvalds/linux@e8c07082a810f). We've always overridden that with gnu99
because we use some newer features.

More recent kernels are using C11 features in headers that we include.
GCC generally doesn't seem to care, but more recent versions of Clang
seem to be enforcing our gnu99 override more strictly, which breaks the
build in some configurations.

Just bumping our "override" to match the kernel seems to be the easiest
workaround. It's an effective no-op since 5.18, while still allowing us
to build on older kernels.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Alexander Motin <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Rob Norris <[email protected]>
Closes #17954

Author: robn

module/Kbuild.in

@@ -2,7 +2,7 @@
 # first.  This ensures its module initialization function is run before
 # any of the other module initialization functions which depend on it.
 
-ZFS_MODULE_CFLAGS += -std=gnu99 -Wno-declaration-after-statement
+ZFS_MODULE_CFLAGS += -std=gnu11 -Wno-declaration-after-statement
 ZFS_MODULE_CFLAGS += -Wmissing-prototypes
 ZFS_MODULE_CFLAGS += @KERNEL_DEBUG_CFLAGS@ @KERNEL_NO_FORMAT_ZERO_LENGTH@