fs: load the ->i_sb pointer once in inode_sb_list_{add,del}

fs: load the ->i_sb pointer once in inode_sb_list_{add,del}

While this may sound like a pedantic clean up, it does in fact impact
code generation -- the patched add routine is slightly smaller.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20250319004635.1820589-1-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>