config: Fix parsing of mount options

config: Fix parsing of mount options

When parsing mount options e.g. from lxc.mount.entry the specified
options are mapped to the flags constants. To do so, the strings
are compared to the options contained in mount_opt. However,
when comparing the strings, the length of the string is not
checked. That entails that the option "rootcontext=selinux-context"
is mapped to the mount option read-only (ro). This commit fixes
this issue by checking if a '=' is contained in the specified option
and additionally comparing the length of the strings.

Signed-off-by: Maximilian Blenk <Maximilian.Blenk@bmw.de>