driver: iio: add missing checks on iio_info's callback access

In the Linux kernel, the following vulnerability has been resolved:

driver: iio: add missing checks on iio_info's callback access

Some callbacks from iio_info structure are accessed without any check, so

if a driver doesn't implement them trying to access the corresponding

sysfs entries produce a kernel oops such as:

[ 2203.527791] Unable to handle kernel NULL pointer dereference at virtual address 00000000 when execute

[...]

[ 2203.783416] Call trace:

[ 2203.783429] iio_read_channel_info_avail from dev_attr_show+0x18/0x48

[ 2203.789807] dev_attr_show from sysfs_kf_seq_show+0x90/0x120

[ 2203.794181] sysfs_kf_seq_show from seq_read_iter+0xd0/0x4e4

[ 2203.798555] seq_read_iter from vfs_read+0x238/0x2a0

[ 2203.802236] vfs_read from ksys_read+0xa4/0xd4

[ 2203.805385] ksys_read from ret_fast_syscall+0x0/0x54

[ 2203.809135] Exception stack(0xe0badfa8 to 0xe0badff0)

[ 2203.812880] dfa0: 00000003 b6f10f80 00000003 b6eab000 00020000 00000000

[ 2203.819746] dfc0: 00000003 b6f10f80 7ff00000 00000003 00000003 00000000 00020000 00000000

[ 2203.826619] dfe0: b6e1bc88 bed80958 b6e1bc94 b6e1bcb0

[ 2203.830363] Code: bad PC value

[ 2203.832695] ---[ end trace 0000000000000000 ]---