File flags handling fixes for ext2fs:

File flags handling fixes for ext2fs:

- Disallow setting of flags not supported by ext2fs.
- Map EXT2_APPEND_FL to SF_APPEND.
- Map EXT2_IMMUTABLE_FL to SF_IMMUTABLE.
- Map EXT2_NODUMP_FL to UF_NODUMP.

Note that ext2fs doesn't support user settable append and immutable flags.
EXT2_NODUMP_FL is an user settable flag also on Linux.

PR:		kern/122047
Reported by:	Ighighi
Submitted by:	Aditya Sarawgi (original version)
Reviewed by:	bde
Approved by:	trasz (mentor)