[PATCH] add vfs_* helpers for xattr operations

Add vfs_getxattr, vfs_setxattr and vfs_removexattr helpers for common checks
around invocation of the xattr methods.  NFSD already was missing some of the
checks and there will be more soon.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Cc: James Morris <jmorris@namei.org>

(James, I haven't touched selinux yet because it's doing various odd things
and I'm not sure how it would interact with the security attribute fallbacks
you added.  Could you investigate whether it could use vfs_getxattr or if not
add a __vfs_getxattr helper to share the bits it is fine with?)

For NFSv4: instead of just converting it add an nfsd_getxattr helper for the
code shared by NFSv2/3 and NFSv4 ACLs.  In fact that code isn't even
NFS-specific, but I'll wait for more users to pop up first before moving it to
common code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Dave Kleikamp <shaggy@austin.ibm.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Neil Brown <neilb@suse.de>
Cc: Trond Myklebust <trond.myklebust@fys.uio.no>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>