fs: fix undefined behavior in bit shift for SB_NOUSER

Shifting signed 32-bit value by 31 bits is undefined, so changing
significant bit to unsigned. It was spotted by UBSAN.

So let's just fix this by using the BIT() helper for all SB_* flags.

Fixes: e462ec50

 ("VFS: Differentiate mount flags (MS_*) from internal superblock flags")
Signed-off-by: Hao Ge <gehao@kylinos.cn>
Message-Id: <20230424051835.374204-1-gehao@kylinos.cn>
[brauner@kernel.org: use BIT() for all SB_* flags]
Signed-off-by: Christian Brauner <brauner@kernel.org>