mmp: do not use O_DIRECT when working with regular file

mmp: do not use O_DIRECT when working with regular file

Currently the mmp block is read using O_DIRECT to avoid any caching that
may be done by the VM. However when working with regular files this
creates alignment issues when the device of the host file system has
sector size larger than the blocksize of the file system in the file
we're working with.

This can be reproduced with t_mmp_fail test when run on the device with
4k sector size because the mke2fs fails when trying to read the mmp
block.

Fix it by disabling O_DIRECT when working with regular files. I don't
think there is any risk of doing so since the file system layer, unlike
shared block device, should guarantee cache consistency.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>