Replace nested ifs in the EVM xattr verification logic with a switch
statement, making the code easier to understand.

Also, add comments to the if statements in the out section and constify the
cause variable.
Signed-off-by: NMimi Zohar <zohar@linux.vnet.ibm.com>
Signed-off-by: NThiago Jung Bauermann <bauerman@linux.vnet.ibm.com>
Acked-by: NSerge Hallyn <serge@hallyn.com>