From: "Jose R. Santos" <jrsantos@austin.ibm.com>

After discussing it with Neil, he fell that the original justification for
taking the kernel_lock on find_exported_dentry() is not longer valid and
should be safe to remove.

This patch fixes an issue while running SpecSFS where under memory
pressure, shrinking dcache cause find_exported_dentry() to allocate
disconnected dentries that later needed to be properly connected.  The
connecting part of the code was done with BKL taken which cause a sharp
drop in performance during iterations and profiles showing 75% time spent
on find_exported_dentry().  After applying the patch, time spent on the
function is reduce to <1%.

I have tested this on an 8-way machine with 56 filesystems for several days
now with no problems using ext2, ext3, xfs and jfs.