fix(dracut-functions.sh): suppress findmnt error msg if /etc/fstab not exist

fix(dracut-functions.sh): suppress findmnt error msg if /etc/fstab not exist

When the rootfs of a virtual machine is virtiofs shared folder,
the /etc/fstab file may not exist. As a result, the "findmnt --fstab"
will complain "findmnt: can't read (null): No such file or directory"
when trying to rebuild kdump initramfs within the vm.

This patch checks if /etc/fstab exist before calling "findmnt --fstab"
to suppress the error message.

Before:
$ kdumpctl rebuild
kdump: Rebuilding /boot/initramfs-5.14.0-182.el9.x86_64kdump.img
findmnt: can't read (null): No such file or directory
findmnt: can't read (null): No such file or directory
findmnt: can't read (null): No such file or directory

After:
$ kdumpctl rebuild
kdump: Rebuilding /boot/initramfs-5.14.0-182.el9.x86_64kdump.img

Signed-off-by: Tao Liu <ltao@redhat.com>