Incorrect handling of UF_IMMUTABLE & UF_APPEND flags on EXT2FS

[Ighighi Ighighi]

Almost 2 months have passed since I submitted this PR through GNATS
(which cc'd it to freebsd-bugs), so I thought that maybe I should forward it
to this list so it gets the attention it deserves:

http://www.freebsd.org/cgi/query-pr.cgi?pr=kern/122047
http://lists.freebsd.org/pipermail/freebsd-bugs/2008-March/029740.html

Some errata notes:
The bug may be present in REISERFS, but there's no write support anyway.

Salutes,
Igh