fastboot: use filename instead of fd to generate filesystem image

mke2fs tool takes a filename and has its own open function to handle
output file. Change fastboot in preparation to switch from make_ext4
to mke2fs.

Test: fastboot format:ext4 userdata
Bug: 35219933
Change-Id: I7a31cb215f443a4a7cb0bfc23ec28c121a6101e6