thunderbolt: debugfs: Fix margining error counter buffer leak

thunderbolt: debugfs: Fix margining error counter buffer leak

When USB4 lane margining debugfs write support is enabled,
margining_error_counter_write() copies the user input with
validate_and_copy_from_user(). This allocates a temporary page that is
only needed while parsing the requested error counter mode.

The function currently returns without freeing that page. This leaks one
page per write to the error_counter debugfs file, including successful
writes and writes that later fail while taking the domain lock or because
software margining is not enabled.

Free the temporary page once parsing has completed, and also before
returning from the invalid-input path.

Fixes: 10904df3f20c ("thunderbolt: Improve software receiver lane margining")
Signed-off-by: Xu Rao <raoxu@uniontech.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>