msdosfs: bound slot offset in mbnambuf_write()

[ojanerif_amd.com]

Summary

mbnambuf_write() only checks the slot offset against nb_buf when it
has to shift an earlier substring. The first substring of a name is
copied without the check, and its id comes from the on-disk winentry.

Do the check unconditionally.

Sponsored by: Advanced Micro Devices, Inc.
Signed-off-by: Osvaldo Janeri Filho <[e-mail]>

Test Plan

Created, remounted and listed names of 1 to 255 characters (ASCII, and
UTF-8 with -L en_US.UTF-8) on a FAT32 md(4) image. fsck_msdosfs -n is
clean, and the fsck_msdosfs and newfs_msdos tests pass.

[imp]

This looks good... but it might be good to have at least a reference FAT image with the mal-formed LFN entry for regression.