fs/buffer: Remove redundant assignment to err

Variable 'err' set but not used.

fs/buffer.c:2613:2: warning: Value stored to 'err' is never read.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=4589


Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>