adding SUN_LEN macro to linux glibc/musl

[glibc ref](https://github.com/bminor/glibc/blob/1b6f868625403d6b7683af840e87d2b18d5d7731/socket/sys/un.h#L41)
[musl ref](https://github.com/bminor/musl/blob/c47ad25ea3b484e10326f933e927c0bc8cded3da/include/sys/un.h#L24)

Author: devnexen

libc-test/semver/linux-gnu.txt

@@ -448,6 +448,7 @@ STA_PPSTIME
 STA_PPSWANDER
 STA_RONLY
 STA_UNSYNC
+SUN_LEN
 ST_RELATIME
 SYSFS_MAGIC
 TCA_CHAIN

libc-test/semver/linux-musl.txt

@@ -36,6 +36,7 @@ RWF_DSYNC
 RWF_HIPRI
 RWF_NOWAIT
 RWF_SYNC
+SUN_LEN
 USER_PROCESS
 _CS_V6_ENV
 _CS_V7_ENV

src/unix/linux_like/linux/mod.rs

@@ -6053,6 +6053,17 @@ f! {
     pub fn ELF64_R_INFO(sym: Elf64_Xword, t: Elf64_Xword) -> Elf64_Xword {
         sym << 32 + t
     }
+
+    #[cfg(target_env = "gnu")]
+    pub fn SUN_LEN(s: crate::sockaddr_un) -> usize {
+        core::mem::offset_of!(crate::sockaddr_un, sun_path) +
+            unsafe { crate::strlen(s.sun_path.as_ptr()) }
+    }
+
+    #[cfg(target_env = "musl")]
+    pub fn SUN_LEN(s: crate::sockaddr_un) -> usize {
+        2 * (unsafe { crate::strlen(s.sun_path.as_ptr()) })
+    }
 }
 
 safe_f! {