random: simplify loop in random_read

random: simplify loop in random_read

The loop condition never changes until just before a break, so we
might as well write it as a constant.  Also since 61a683c30d4d
("random: drop weird m_time/a_time manipulation") we don't do anything
after the loop finishes, so the 'break's might as well return
directly.  Some other simplifications.

There should be no change in behavior introduced by this commit.

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