Fix 'make checkbashisms` warnings

Fix 'make checkbashisms` warnings

The awk command used by the checkbashisms target incorrectly
adds the escape character before the ! and # characters.  This
results in the following warnings because these characters do not
need to be escaped.

    awk: cmd. line:1: warning: regexp escape sequence
        `\!' is not a known regexp operator
    awk: cmd. line:1: warning: regexp escape sequence
        `\#' is not a known regexp operator

Remove the unneeded escape character before ! and #.

Valid escape sequences are:

    https://www.gnu.org/software/gawk/manual/html_node/Escape-Sequences.html

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #11902