Fix regression -spc, -rootdir options were ignored for floppy image made by IMGMAKE command

[maron2000]

Fix -spc, -rootdir options of IMGMAKE command were ignored for floppy image since commit 4462d88.
Fixes #5171

$ file flptest.img
flptest.img: DOS/MBR boot sector, code offset 0x3c+2, OEM-ID "MSDOS5.0", sectors/cluster 4, FAT  1, root entries 64, sectors 2880 (volumes <=32 MB), sectors/FAT 9, sectors/track 18, serial number 0x9544, unlabeled, FAT (12 bit), followed by FAT

[maron2000]

Adjustment of sect_per_fat value was required for floppy images with customized root entries and/or fat copies.
fsck.vfat command now runs without errors even after several files are copied to the floppy image.

IMGMAKE fd1200.ima -t fd_1200 -fatcopies 1 -rootdir 32 -spc 1

$ fsck.vfat -v fd1200.ima
fsck.fat 4.2 (2021-01-31)
Checking we can access the last sector of the filesystem
Boot sector contents:
System ID "MSDOS5.0"
Media byte 0xf9 (3,5" 720k floppy 2s/80tr/9sec or 5.25" 1.2M floppy 2s/80tr/15sec)
       512 bytes per logical sector
       512 bytes per cluster
         1 reserved sector
First FAT starts at byte 512 (sector 1)
         1 FATs, 12 bit entries
      4096 bytes per FAT (= 8 sectors)
Root directory starts at byte 4608 (sector 9)
        32 root directory entries
Data area starts at byte 5632 (sector 11)
      2389 data clusters (1223168 bytes)
15 sectors/track, 2 heads
         0 hidden sectors
      2400 sectors total
Checking for unused clusters.
fd1200.ima: 31 files, 1148/2389 clusters