dm: fix copying after src array boundaries

dm: fix copying after src array boundaries

The blammed commit copied to argv the size of the reallocated argv,
instead of the size of the old_argv, thus reading and copying from
past the old_argv allocated memory.

Following BUG_ON was hit:
[    3.038929][    T1] kernel BUG at lib/string_helpers.c:1040!
[    3.039147][    T1] Internal error: Oops - BUG: 00000000f2000800 [#1]  SMP
...
[    3.056489][    T1] Call trace:
[    3.056591][    T1]  __fortify_panic+0x10/0x18 (P)
[    3.056773][    T1]  dm_split_args+0x20c/0x210
[    3.056942][    T1]  dm_table_add_target+0x13c/0x360
[    3.057132][    T1]  table_load+0x110/0x3ac
[    3.057292][    T1]  dm_ctl_ioctl+0x424/0x56c
[    3.057457][    T1]  __arm64_sys_ioctl+0xa8/0xec
[    3.057634][    T1]  invoke_syscall+0x58/0x10c
[    3.057804][    T1]  el0_svc_common+0xa8/0xdc
[    3.057970][    T1]  do_el0_svc+0x1c/0x28
[    3.058123][    T1]  el0_svc+0x50/0xac
[    3.058266][    T1]  el0t_64_sync_handler+0x60/0xc4
[    3.058452][    T1]  el0t_64_sync+0x1b0/0x1b4
[    3.058620][    T1] Code: f800865e a9bf7bfd 910003fd 941f48aa (d4210000)
[    3.058897][    T1] ---[ end trace 0000000000000000 ]---
[    3.059083][    T1] Kernel panic - not syncing: Oops - BUG: Fatal exception

Fix it by copying the size of src, and not the size of dst, as it was.

Fixes: 5a2a6c428190 ("dm: always update the array size in realloc_argv on success")
Cc: stable@vger.kernel.org
Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>