vfs: names up to 255 bytes and paths up to 4096, like Linux

Every path component was copied into a DIRSIZ buffer and every path
into a MAXPATH one, so names were cut at 29 bytes and paths at 127
without an error. Two names sharing their first 29 bytes opened the
same file, on SchFS and FAT alike, though both hold 255.

VFS_NAME_MAX (255) and VFS_PATH_MAX (4096) are the limits now, and
anything longer is ENAMETOOLONG. namex_from() takes its path buffers
from kalloc(), a page each, rather than putting two of them on an 8 KB
kernel stack. The v1 format still stores DIRSIZ bytes, and refuses a
longer name instead of matching on its first 30.

Checked: 255-byte names on SchFS and FAT (fsck.fat clean, and the FAT
name answers to its uppercase form), 256 refused, a 200-byte path and a
link to it, and no pages lost over 700 lookups through links.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

kernel/fs/fat/fat.c

@@ -667,7 +667,7 @@ static int
 vfs_name(const char *name, uint16 *out)
 {
   int n = 0;
-  while (n < DIRSIZ && name[n])
+  while (n < VFS_NAME_MAX && name[n])
     n++;
   int len = utf8_to_16(name, n, out);
   if (len < 0)