Fix size of dirent buffer on illumos (#104448)

---------

Co-authored-by: Adeel Mujahid <3840695+am11@users.noreply.github.com>

Author: AustinWise

src/native/libs/System.Native/pal_io.c

@@ -54,6 +54,10 @@
 #endif
 #endif
 
+#ifdef TARGET_SUNOS
+#include <sys/param.h>
+#endif
+
 #ifdef _AIX
 #include <alloca.h>
 // Somehow, AIX mangles the definition for this behind a C++ def
@@ -436,10 +440,17 @@ static const size_t dirent_alignment = 8;
 int32_t SystemNative_GetReadDirRBufferSize(void)
 {
 #if HAVE_READDIR_R
+    size_t result = sizeof(struct dirent);
+#ifdef TARGET_SUNOS
+    // The d_name array is declared with only a single byte in it.
+    // We have to add pathconf("dir", _PC_NAME_MAX) more bytes.
+    // MAXNAMELEN is the largest possible value returned from pathconf.
+    result += MAXNAMELEN;
+#endif
     // dirent should be under 2k in size
-    assert(sizeof(struct dirent) < 2048);
+    assert(result < 2048);
     // add some extra space so we can align the buffer to dirent.
-    return sizeof(struct dirent) + dirent_alignment - 1;
+    return (int32_t)(result + dirent_alignment - 1);
 #else
     return 0;
 #endif