usb: gadget: configfs: Fix use-after-free issue with udc_name

There is a use-after-free issue, if access udc_name
in function gadget_dev_desc_UDC_store after another context
free udc_name in function unregister_gadget.

Context 1:
gadget_dev_desc_UDC_store()->unregister_gadget()->
free udc_name->set udc_name to NULL

Context 2:
gadget_dev_desc_UDC_show()-> access udc_name

Call trace:
dump_backtrace+0x0/0x340
show_stack+0x14/0x1c
dump_stack+0xe4/0x134
print_address_description+0x78/0x478
__kasan_report+0x270/0x2ec
kasan_report+0x10/0x18
__asan_report_load1_noabort+0x18/0x20
string+0xf4/0x138
vsnprintf+0x428/0x14d0
sprintf+0xe4/0x12c
gadget_dev_desc_UDC_show+0x54/0x64
configfs_read_file+0x210/0x3a0
__vfs_read+0xf0/0x49c
vfs_read+0x130/0x2b4
SyS_read+0x114/0x208
el0_svc_naked+0x34/0x38

Add mutex_lock to protect this kind of scenario.

Signed-off-by: Eddie Hung <eddie.hung@mediatek.com>
Signed-off-by: Macpaul Lin <macpaul.lin@mediatek.com>
Reviewed-by: Peter Chen <peter.chen@nxp.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/1609239215-21819-1-git-send-email-macpaul.lin@mediatek.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
