api.c: use /proc/self/mounts

api.c: use /proc/self/mounts

Use /proc/self/mount instead of /proc/mounts, which is a symbolic link
to the former. The symbolic link creation can be traced back to Linux
Kernel commit 59c7572e82d6 ("proc: remove fs/proc/proc_misc.c"). Also,
the /proc/mounts are not available in the cases where the /proc is
mounted with subset=pid option.

As per kernel docs filesystems/procs.rst:
"subset=pid hides all top level files and directories in the procfs that
are not related to tasks."

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>
(cherry picked from commit 16be308b037bbe11a54b692c227e160f1132eabe)