ofpbuf: Add helper method to truncate the buffer. #6578

Job	Run time
libreswan (ubuntu-24.04)	4s
linux rpm fedora	4m 12s
linux deb no dpdk	5m 12s
dpdk gcc (ubuntu-24.04)	8s
build oss-fuzz fuzzers	2m 53s
osx clang --disable-ssl	1m 33s
clang-analyze	20m 29s
linux gcc --disable-ssl	2m 48s
linux clang --disable-ssl	4m 44s
linux gcc c99	2m 53s
linux clang c99	2m 33s
linux gcc test	10m 26s
linux clang address,undefined test	16m 47s
linux gcc test --enable-shared	12m 32s
linux clang test --enable-shared	10m 59s
linux gcc check check-dpdk dpdk	17m 2s
linux clang check check-dpdk dpdk	18m 21s
linux gcc test -ljemalloc	9m 4s
linux clang test -ljemalloc	10m 47s
linux gcc --enable-afxdp	2m 46s
linux clang --enable-afxdp	2m 49s
linux gcc dpdk --enable-shared	4m 1s
linux clang dpdk --enable-shared	3m 43s
linux gcc dpdk-shared	3m 9s
linux clang dpdk-shared	3m 9s
linux gcc dpdk-shared --enable-shared	3m 59s
linux clang dpdk-shared --enable-shared	5m 12s
linux gcc m32 --disable-ssl	3m 6s
linux gcc check-ovsdb-cluster	6m 48s
linux gcc check-kernel -100	10m 23s
linux gcc check-kernel 100-	15m 8s
linux clang address,undefined check-kernel -100	12m 49s
linux clang address,undefined check-kernel 100-	17m 34s
linux gcc check-offloads -100	9m 25s
linux gcc check-offloads 100-	11m 28s
linux gcc dpdk check-system-userspace	10m 50s
linux clang address,undefined dpdk check-system-userspace	14m 29s
linux gcc dpdk check-system-tso	10m 43s
linux gcc dpdk check-afxdp	11m 26s
	5h 16m 24s

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