MFC r339816: mount_msdosfs

MFC r339816: mount_msdosfs

  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

git-svn-id: svn://svn.freebsd.org/base/stable/10@340954 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f