libext2fs: refactor Direct I/O alignment requirement calculations

libext2fs: refactor Direct I/O alignment requirement calculations

Create a new function, ext2fs_get_dio_alignment(), which returns the
alignment requirements for direct I/O.  This way we can factor out the
code from MMP and the Unix I/O manager.  The two modules weren't
consistently calculating the alignment factors, and in particular MMP
would sometimes use a larger alignment factor than was strictly
necessary.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>