build-sys: require libmount >= 2.30 (#6795)

Fixes #4871.

The new libmount has two changes relevant for us:

- x-* options are propagated to /run/mount/utab and are visible through
  libmount (fixes #4817).

- umount -c now really works (partially solves #6115).

README

@@ -131,9 +131,8 @@ REQUIREMENTS:
 
         glibc >= 2.16
         libcap
-        libmount >= 2.27.1 (from util-linux)
-                (util-linux < 2.29 *must* be built with --enable-libmount-force-mountinfo,
-                 and later versions without --enable-libmount-support-mtab.)
+        libmount >= 2.30 (from util-linux)
+                (util-linux *must* be built without --enable-libmount-support-mtab)
         libseccomp >= 2.3.1 (optional)
         libblkid >= 2.24 (from util-linux) (optional)
         libkmod >= 15 (optional)

meson.build

@@ -659,7 +659,7 @@ if not libcap.found()
 endif
 
 libmount = dependency('mount',
-                      version : '>= 2.27')
+                      version : '>= 2.30')
 
 want_seccomp = get_option('seccomp')
 if want_seccomp != 'false'