libext2fs: factor out I/O buffer allocation

libext2fs: factor out I/O buffer allocation

Create a new function, io_channel_alloc_buf() which allocates I/O
buffers with appropriate alignment if we are using direct I/O.  The
original code was sometimes using a larger alignment factor than
necessary, and would always request an aligned memory buffer even when
it was not necessary since the block device was not opened with
O_DIRECT.

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