umount: Make sure exit code does not overflow

POSIX exit code is only 8-bit, and since umount sums up error
codes, it can sometimes report success (exit code 0) even though
a number of operations failed.

For example, running, in an empty directory:
 umount `seq 1 7`
returns 224 (7*32), since none of the 7 mount point exists but
 umount `seq 1 8`
returns 0 (8*32=256)

This patch clips the return value to 255.

Signed-off-by: Nicolas Boichat <nicolas@boichat.ch>