ext4: Define a new set of flags for ext4_get_blocks()

The functions ext4_get_blocks(), ext4_ext_get_blocks(), and
ext4_ind_get_blocks() used an ad-hoc set of integer variables used as
boolean flags passed in as arguments.  Use a single flags parameter
and a setandard set of bitfield flags instead.  This saves space on
the call stack, and it also makes the code a bit more understandable.

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