mke2fs: Allow specifying reserved_ratio via mke2fs.conf

mke2fs: Allow specifying reserved_ratio via mke2fs.conf

This patch adds support for specifying 'reserved_ratio' (percent blocks
reserved for super user, same as '-m' command line option) in mke2fs.conf.
It adds profile_get_double function in profile.c that allows reading
floating point values from profile files.

Signed-off-by: Aditya Kali <adityakali@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>