sdevent:inode entry is not released after creating dump

For every dump generated an "anon_inode:[pidfd]" entry is
created for the associated process and the same is not
released.

When the number of inode entries reach the maximum count
dump request will fail and the dump service takes a reset
with a core dump.

When a dump is requested a child process is spawned to
perform dump collection, parent process waits on the
child process completion using sd-event-add-child
systemd call.

sd-event-add-child adds an inode entry which needs to
be released during the callback method called after
child process exit using unref but the same is not done.

Now switching to sdeventplus::source::Child wrapper class for
sd-event-add-child which takes care of releasing the inode fd.

Tested:
'''
Without the fix for the 3 dumps created noticed 3 anon_inode:[pidfd]
are added but not cleared
root@rain135bmc:/proc/2044/fd# ls -la
dr-x------    2 root     root             0 Jun 27 08:38 .
dr-xr-xr-x    8 root     root             0 Jun 27 08:38 ..
lr-x------    1 root     root            64 Jun 27 08:39 0 -> /dev/null
lrwx------    1 root     root            64 Jun 27 08:39 1 -> socket:[21586]
lrwx------    1 root     root            64 Jun 27 08:39 10 ->anon_inode:[timerfd]
lrwx------    1 root     root            64 Jun 27 08:47 11 ->anon_inode:[pidfd]
lrwx------    1 root     root            64 Jun 27 08:47 12 ->anon_inode:[pidfd]
lrwx------    1 root     root            64 Jun 27 08:47 13 ->anon_inode:[pidfd]

After the fix noticed all the anon_inode:[pidfd] are cleared
dr-x------    2 root     root             0 Jun 27 08:38 .
dr-xr-xr-x    8 root     root             0 Jun 27 08:38 ..
lr-x------    1 root     root            64 Jun 27 08:39 0 -> /dev/null
lrwx------    1 root     root            64 Jun 27 08:39 1 -> socket:[21586]
lrwx------    1 root     root            64 Jun 27 08:39 10 ->anon_inode:[timerfd]
'''

Signed-off-by: Marri Devender Rao <devenrao@in.ibm.com>
Change-Id: I9a2fec0a2287bfe978b811671c1cfeb1356e59d2
diff --git a/meson.build b/meson.build
index 3566a89..75b57bf 100644
--- a/meson.build
+++ b/meson.build
@@ -27,6 +27,7 @@
 sdbusplus_dep = dependency('sdbusplus')
 sdbusplusplus_prog = find_program('sdbus++')
 sdbuspp_gen_meson_prog = find_program('sdbus++-gen-meson')
+sdeventplus_dep = dependency('sdeventplus')
 
 phosphor_dbus_interfaces_dep = dependency('phosphor-dbus-interfaces')
 phosphor_logging_dep = dependency('phosphor-logging')
@@ -177,6 +178,7 @@
 phosphor_dump_manager_dependency = [
         phosphor_dbus_interfaces_dep,
         sdbusplus_dep,
+        sdeventplus_dep,
         phosphor_logging_dep,
         fmt_dep,
         cereal_dep,