fs: define inode flags using bit numbers

Define the VFS inode flags using bit numbers instead of hardcoding
powers of 2, which has become unwieldy now that we're up to 65536.

No change in the actual values.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>