Fix potential 2**32-1 overflow by using e2p_percent()

Fix potential 2**32-1 overflow by using e2p_percent()

Add a new functiom, e2p_percent(), which correct calculates the percentage
of a number based on a given percentage, without worrying about overflow
issues.  This is used where we calculate the number of reserved blocks using
a percentage of the total number of blocks in a filesystem.

Based on patches from Eric Sandeen, but generalized to use this new function.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Eric Sandeen <esandeen@redhat.com>