iio: buffer: check return value of kstrdup_const()

Check return value of kstrdup_const() in iio_buffer_wrap_attr(),
or it will cause null-ptr-deref in kernfs_name_hash() when calling
device_add() as follows:

BUG: kernel NULL pointer dereference, address: 0000000000000000
RIP: 0010:strlen+0x0/0x20
Call Trace:
 kernfs_name_hash+0x22/0x110
 kernfs_find_ns+0x11d/0x390
 kernfs_remove_by_name_ns+0x3b/0xb0
 remove_files.isra.1+0x7b/0x190
 internal_create_group+0x7f1/0xbb0
 internal_create_groups+0xa3/0x150
 device_add+0x8f0/0x2020
 cdev_device_add+0xc3/0x160
 __iio_device_register+0x1427/0x1b40 [industrialio]
 __devm_iio_device_register+0x22/0x80 [industrialio]
 adjd_s311_probe+0x195/0x200 [adjd_s311]
 i2c_device_probe+0xa07/0xbb0

Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: 15097c7a ("iio: buffer: wrap all buffer attributes into iio_dev_attr")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20211013040438.1689277-1-yangyingliang@huawei.com


Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>