ext4: improve handling of conflicting mount options

If the user explicitly specifies conflicting mount options for
delalloc or dioread_nolock and data=journal, fail the mount, instead
of printing a warning and continuing (since many user's won't look at
dmesg and notice the warning).

Also, print a single warning that data=journal implies that delayed
allocation is not on by default (since it's not supported), and
furthermore that O_DIRECT is not supported.  Improve the text in
Documentation/filesystems/ext4.txt so this is clear there as well.

Similarly, if the dioread_nolock mount option is specified when the
file system block size != PAGE_SIZE, fail the mount instead of
printing a warning message and ignoring the mount option.

Change-Id: I936c5bbe42d1da494a3c5cee8d2f216848b2a9e7
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>