usb: gadget: uac: validate rate list length before storing

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

usb: gadget: uac: validate rate list length before storing

UAC1 and UAC2 configfs rate-list attributes parse a comma-separated
list of sampling rates and store each parsed value in fixed-size arrays.
The arrays have UAC_MAX_RATES entries, but the store paths do not check
that the input contains at most that many tokens before writing through
opts->name##s[i++].

Writing more than ten rates therefore writes past the end of the
p_srates[] or c_srates[] array in struct f_uac1_opts or struct
f_uac2_opts.

With CONFIG_UBSAN_BOUNDS enabled, writing an 11-entry rate list to the
UAC1 p_srate attribute reports:

UBSAN: array-index-out-of-bounds
drivers/usb/gadget/function/f_uac1.c:1669:1
index 10 is out of range for type 'int [10]'
__ubsan_handle_out_of_bounds.cold
f_uac1_opts_p_srate_store
configfs_write_iter
vfs_write
ksys_write
do_syscall_64

The same reproducer against the UAC2 p_srate attribute reports:

UBSAN: array-index-out-of-bounds
drivers/usb/gadget/function/f_uac2.c:2087:1
index 10 is out of range for type 'int [10]'
__ubsan_handle_out_of_bounds.cold
f_uac2_opts_p_srate_store
configfs_write_iter
vfs_write
ksys_write
do_syscall_64

Reject additional tokens once UAC_MAX_RATES entries have been parsed.
Also keep the original kstrdup() pointer for kfree(), because strsep()
advances the parsing cursor. Freeing the advanced cursor leaks the
original buffer on successful parses and can free an interior pointer on
some error paths.