sysv: return 'err' instead of 0 in __sysv_write_inode

Fixes gcc '-Wunused-but-set-variable' warning:

fs/sysv/inode.c: In function '__sysv_write_inode':
fs/sysv/inode.c:239:6: warning:
 variable 'err' set but not used [-Wunused-but-set-variable]

__sysv_write_inode should return 'err' instead of 0

Fixes: 05459ca8 ("repair sysv_write_inode(), switch sysv to simple_fsync()")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>