erofs: simplify erofs_xattr_generic_get()

erofs: simplify erofs_xattr_generic_get()

erofs_xattr_generic_get() won't be called from xattr handlers other than
user/trusted/security xattr handler, and thus there's no need of extra
checking.

Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230330082910.125374-4-jefflexu@linux.alibaba.com
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>