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/fat.md

@@ -92,9 +92,6 @@ directory ends at the first entry whose first byte is zero.
 
 ## What it does not do
 
-- **Names longer than 29 bytes.** FAT allows 255 UTF-16 units, but the VFS's
-  `DIRSIZ` caps a path component, so a longer name is cut short on the way in.
-  Lifting that is a VFS change.
 - **Permissions and owners.** Every file shows as `0755 root:root`, like Linux
   vfat with its defaults. The read-only attribute is kept but not enforced.
 - **exFAT.** Large SD cards (SDXC, over 32 GB) ship with it. It is a different