ofpbuf: Add helper method to truncate the buffer. #2567

Job	Run time
libreswan (ubuntu-24.04)	6s
linux deb no dpdk	5m 10s
linux rpm fedora	4m 2s
dpdk gcc (ubuntu-24.04)	5s
build oss-fuzz fuzzers	2m 53s
osx clang --disable-ssl	1m 47s
clang-analyze-cache	19s
linux gcc --disable-ssl	2m 42s
linux clang --disable-ssl	2m 44s
linux gcc c99	2m 36s
linux clang c99	2m 43s
linux gcc test	8m 20s
linux clang address,undefined test	16m 26s
linux gcc test --enable-shared	11m 30s
linux clang test --enable-shared	10m 55s
linux gcc check check-dpdk dpdk	18m 51s
linux clang check check-dpdk dpdk	18m 56s
linux gcc test -ljemalloc	9m 20s
linux clang test -ljemalloc	9m 15s
linux gcc --enable-afxdp	2m 48s
linux clang --enable-afxdp	2m 34s
linux gcc dpdk --enable-shared	3m 58s
linux clang dpdk --enable-shared	3m 46s
linux gcc dpdk-shared	3m 5s
linux clang dpdk-shared	2m 57s
linux gcc dpdk-shared --enable-shared	5m 2s
linux clang dpdk-shared --enable-shared	3m 55s
linux gcc m32 --disable-ssl	2m 59s
linux gcc check-ovsdb-cluster	6m 55s
linux gcc check-kernel -100	10m 39s
linux gcc check-kernel 100-	16m 52s
linux clang address,undefined check-kernel -100	13m 12s
linux clang address,undefined check-kernel 100-	19m 14s
linux gcc check-offloads -100	10m 0s
linux gcc check-offloads 100-	13m 12s
linux gcc dpdk check-system-userspace	12m 20s
linux clang address,undefined dpdk check-system-userspace	16m 12s
linux gcc dpdk check-system-tso	12m 1s
linux gcc dpdk check-afxdp	13m 36s
clang-analyze	10m 47s
	5h 14m 44s

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