Improve F5 template /usr partition trigger

Please consider to improve template.

• The template name is "F5 Big-IP SNMP" (https://git.zabbix.com/projects/ZBX/repos/zabbix/browse/templates/net/f5_bigip_snmp?at=release/6.0)
• The discovery rule is "File system discovery"
• The trigger prototype is "F5 BIG-IP: Low free space in file system {#PART.NAME}"
• The user macro that holds the threshold value is "{$BIGIP.FS.FREE.WARN.MIN}"
• The problem event on low free disk space should not be at all generated for the "/usr" filesystem due to the fact that this filesystem is generated automatically and is read-only (https://support.f5.com/csp/article/K23607394)