iio: gts-helper: Fix memory leaks in iio_gts_build_avail_scale_table()

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

iio: gts-helper: Fix memory leaks in iio_gts_build_avail_scale_table()

modprobe iio-test-gts and rmmod it, then the following memory leak
occurs:

unreferenced object 0xffffff80c810be00 (size 64):
comm "kunit_try_catch", pid 1654, jiffies 4294913981
hex dump (first 32 bytes):
02 00 00 00 08 00 00 00 20 00 00 00 40 00 00 00 ........ ...@...
80 00 00 00 00 02 00 00 00 04 00 00 00 08 00 00 ................
backtrace (crc a63d875e):
[<0000000028c1b3c2>] kmemleak_alloc+0x34/0x40
[<000000001d6ecc87>] __kmalloc_noprof+0x2bc/0x3c0
[<00000000393795c1>] devm_iio_init_iio_gts+0x4b4/0x16f4
[<0000000071bb4b09>] 0xffffffdf052a62e0
[<000000000315bc18>] 0xffffffdf052a6488
[<00000000f9dc55b5>] kunit_try_run_case+0x13c/0x3ac
[<00000000175a3fd4>] kunit_generic_run_threadfn_adapter+0x80/0xec
[<00000000f505065d>] kthread+0x2e8/0x374
[<00000000bbfb0e5d>] ret_from_fork+0x10/0x20
unreferenced object 0xffffff80cbfe9e70 (size 16):
comm "kunit_try_catch", pid 1658, jiffies 4294914015
hex dump (first 16 bytes):
10 00 00 00 40 00 00 00 80 00 00 00 00 00 00 00 ....@...........
backtrace (crc 857f0cb4):
[<0000000028c1b3c2>] kmemleak_alloc+0x34/0x40
[<000000001d6ecc87>] __kmalloc_noprof+0x2bc/0x3c0
[<00000000393795c1>] devm_iio_init_iio_gts+0x4b4/0x16f4
[<0000000071bb4b09>] 0xffffffdf052a62e0
[<000000007d089d45>] 0xffffffdf052a6864
[<00000000f9dc55b5>] kunit_try_run_case+0x13c/0x3ac
[<00000000175a3fd4>] kunit_generic_run_threadfn_adapter+0x80/0xec
[<00000000f505065d>] kthread+0x2e8/0x374
[<00000000bbfb0e5d>] ret_from_fork+0x10/0x20
......

It includes 5*5 times "size 64" memory leaks, which correspond to 5 times
test_init_iio_gain_scale() calls with gts_test_gains size 10 (10*size(int))
and gts_test_itimes size 5. It also includes 5*1 times "size 16"
memory leak, which correspond to one time __test_init_iio_gain_scale()
call with gts_test_gains_gain_low size 3 (3*size(int)) and gts_test_itimes
size 5.

The reason is that the per_time_gains[i] is not freed which is allocated in
the "gts->num_itime" for loop in iio_gts_build_avail_scale_table().