buffers: fix max headroom size

Commit

    f7877d ("Implement DATA_V3 protocol features")

has added support for 64bit pkitd. This means that
headroom for TX buffer should be also increased by 4 bytes.

Signed-off-by: Lev Stipakov <[email protected]>

bufferpool.cpp

@@ -27,7 +27,7 @@
 #include "bufferpool.h"
 #include "trace.h"
 
-#define OVPN_BUFFER_HEADROOM 26 // we prepend TCP packet size (2 bytes) and crypto overhead (24 bytes)
+#define OVPN_BUFFER_HEADROOM 30 // we prepend TCP packet size (2 bytes) and max crypto overhead (28 bytes)
 
 // good enough limit for in-flight packets
 constexpr auto MAX_POOL_SIZE = 100'000;