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

docs/architecture/filesystem.md

@@ -288,6 +288,20 @@ held alive by an `nlink` bump.
 Renaming a *directory* is still refused with `EISDIR`, and so is replacing one:
 both need the `..` fixup and the ancestor-loop check that are not written.
 
+## Names and paths
+
+A name is at most 255 bytes and a path 4096 including its NUL, Linux's
+`NAME_MAX` and `PATH_MAX` (`VFS_NAME_MAX` and `VFS_PATH_MAX` in
+`include/file.h`). Longer is `ENAMETOOLONG`. A filesystem may hold less: the v1
+format stores `DIRSIZ` bytes and refuses longer names rather than matching on
+their first 30.
+
+Both limits used to be the copy buffers: every component went through a
+`DIRSIZ` array and every path through a `MAXPATH` one, so a name was cut at 29
+bytes and a path at 127, silently. Two files sharing their first 29 bytes were
+one file. The path buffers are a page each now, from `kalloc()`, since two of
+them would be most of a kernel stack.
+
 ## Symlinks
 
 Followed the way Linux follows them, in `namex_from()`. A link in the middle of