mount_msdosfs: do not fail mounts requiring locale name conversion table
that is already present in a kernel statically.

For example, the command "mount_msdosfs -L ru_RU.KOI8-R" fails with error
"mount_msdosfs: msdosfs_iconv: File exists" for a kernel having
options LIBICONV and MSDOSFS_ICONV. After this change, it mounts successfully.

MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D16951