mke2fs: fix creating a file system image w/o a pre-existing file

mke2fs: fix creating a file system image w/o a pre-existing file

The mke2fs program should allow creating a file system image when an
explicit file system size is specified, even if the file doesn't yet
exist.  By deferring the call to check_plausible() in commit
942b00cb9d2f ("mke2fs: do not warn about a pre-existing partition
table when using a non-zero offset") this behaviour was broken.

Fix this regression by explicitly creating the file if the file system
size is specified.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>