debugfs: silence build warning and remove incorrect error message

debugfs: silence build warning and remove incorrect error message

Commit fe56188b07 modified the code used to check the block number
argument to the command line -s switch, adding a call to com_err().
This causes a compile time warning because the third argument to
com_err() isn't a string.  Also, when the block number argument is
bad it outputs an incorrect error message - "Operation not permitted".

Fix these minor problems by removing the call to com_err().  Other
code provides enough error reporting information in this case.

Signed-off-by: Eric Whitney <enwlinux@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>