binder: fix unused alloc->free_async_space

commit c6d05e07 upstream.

Each transaction is associated with a 'struct binder_buffer' that stores
the metadata about its buffer area. Since commit 74310e06 ("android:
binder: Move buffer out of area shared with user space") this struct is
no longer embedded within the buffer itself but is instead allocated on
the heap to prevent userspace access to this driver-exclusive info.

Unfortunately, the space of this struct is still being accounted for in
the total buffer size calculation, specifically for async transactions.
This results in an additional 104 bytes added to every async buffer
request, and this area is never used.

This wasted space can be substantial. If we consider the maximum mmap
buffer space of SZ_4M, the driver will reserve half of it for async
transactions, or 0x200000. This area should, in theory, accommodate up
to 262,144 buffers of the minimum 8-byte size. However, after adding
the extra 'sizeof(struct binder_buffer)', the total number of buffers
drops to only 18,724, which is a sad 7.14% of the actual capacity.

This patch fixes the buffer size calculation to enable the utilization
of the entire async buffer space. This is expected to reduce the number
of -ENOSPC errors that are seen on the field.

Fixes: 74310e06

 ("android: binder: Move buffer out of area shared with user space")
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20231201172212.1813387-6-cmllamas@google.com
[cmllamas: fix trivial conflict with missing 261e7818

.]
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>