Fix shellcheck v0.4.6 warnings

Resolve new warnings reported after upgrading to shellcheck
version 0.4.6. This patch contains no functional changes.

• egrep is non-standard and deprecated. Use grep -E instead. [SC2196]
• Check exit code directly with e.g. 'if mycmd;', not indirectly with $?. [SC2181] Suppressed.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7040