nfsd: fix double-locks of directory mutex

A number of nfsd operations depend on the i_mutex to cover more code
than just the fsync, so the approach of 4c728ef5

 "add a vfs_fsync
helper" doesn't work for nfsd.  Revert the parts of those patches that
touch nfsd.

Note: we can't, however, remove the logic from vfs_fsync that was needed
only for the special case of nfsd, because a vfs_fsync(NULL,...) call
can still result indirectly from a stackable filesystem that was called
by nfsd.  (Thanks to Christoph Hellwig for pointing this out.)

Reported-by: Eric Sesterhenn <snakebyte@gmx.de>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>