userlandfs: Ensure room for requests in port

* Add a parameter to RequestAllocator::AllocateAddress and ::AllocateData
  that allows the client to specify a minimum amount of free space that
  must remain in the port buffer.
* Make use of the new parameter in some operations that can fail
  without it.

The Port buffer can be used to store data associated with a Request.
For some file system operations, further requests must be sent through
the port (by calling AllocateRequest) after reserving port buffer
space for data. Unlike AllocateAddress and AllocateData, which can
use an area if the data is larger than the port buffer capacity,
AllocateRequest can only allocate space in the port buffer. If data
previously allocated in the port buffer happens to be large enough to
fill it, then these further AllocateRequest calls will fail.

Change-Id: If03e0afdfbd9fbc36f0e1a04b5d0a20031932b91
Reviewed-on: https://review.haiku-os.org/c/haiku/+/8866
Reviewed-by: waddlesplash <[email protected]>
Tested-by: Commit checker robot <[email protected]>

headers/private/userlandfs/private/RequestAllocator.h

@@ -37,10 +37,10 @@ public:
 
 			status_t			AllocateAddress(Address& address, int32 size,
 									int32 align, void** data,
-									bool deferredInit = false);
+									bool deferredInit = false, int32 reserveForNextRequests = 0);
 			status_t			AllocateData(Address& address, const void* data,
 									int32 size, int32 align,
-									bool deferredInit = false);
+									bool deferredInit = false, int32 reserveForNextRequests = 0);
 			status_t			AllocateString(Address& address,
 									const char* data,
 									bool deferredInit = false);