reiserfs: switch to generic_{get,set,remove}xattr()

reiserfs_xattr_[sg]et() will fail with -EOPNOTSUPP for V1 inodes anyway,
and all reiserfs instances of ->[sg]et() call it and so does ->set_acl().

Checks for name length in the instances had been bogus; they should've
been "bugger off if it's _exactly_ the prefix" (as generic would
do on its own) and not "bugger off if it's shorter than the prefix" -
that can't happen.

xattr_full_name() is needed to adjust for the fact that generic instances
will skip the prefix in the name passed to ->[gs]et(); reiserfs homegrown
analogues didn't.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>