USB: usbfs: Use a spinlock instead of atomic accesses to tally used memory.

While the existing code code imposes a limit on the used memory, it might be
over pessimistic (even if this is unlikely).

Example scenario:
8 threads running in parallel, all entering
"usbfs_increase_memory_usage()" at the same time.
The atomic accesses in "usbfs_increase_memory_usage()" could be
serialized like this:
  8 x "atomic64_add"
  8 x "atomic64_read"
If the 8 x "atomic64_add" raise "usbfs_memory_usage" above the limit,
then all 8 calls of "usbfs_increase_memory_usage()" will return with
-ENOMEM.  If you instead serialize over the whole access to
"usbfs_memory_usage" by using a spinlock, some of these calls will
succeed.

Acked-by: Alan Stern <[email protected]>
Signed-off-by: Ingo Rohloff <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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