USB: core: Use krealloc() in usb_cache_string()

USB: core: Use krealloc() in usb_cache_string()

Instead of "shrinking" the allocation by kmalloc()ing a new, smaller
buffer, utilize krealloc() to shrink the existing allocation. This saves
a memcpy(), as well as eliminates the temporary `smallbuf` allocation,
which guards against allocation failure under extreme memory pressure.

Signed-off-by: Bence Csókás <bence98@sch.bme.hu>
Link: https://patch.msgid.link/20260315-usb-krealloc-v2-1-32f83e090409@sch.bme.hu
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>