msdosfs: fix rename

Use the same locking algorithm for msdosfs_rename() as used by ufs_rename().
Convert doscheckpath() to non-sleeping version.

Reported by: trasz
PR: 257522
In collaboration with: pho
Reviewed by: mckusick
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D31464