Revert "client: invalidate kernel dentries one by one"

This reverts commit 491da5173fc33fec944865f0e2e9a506ec71fc5b.

Since 3.18 kernel, d_invalidate() unhash dentry even when it's
in use. If dentry of process' working directory is invalidated,
calling getcwd(3) will return -ENOENT.

Signed-off-by: Yan, Zheng <zyan@redhat.com>
client: cleanup client callback registration

Signed-off-by: Yan, Zheng <zyan@redhat.com>
client: use remount to trim kernel dcache

when remounting a file system, linux kernel trims all unused dentry
in the file system.

Fixes: #10277
Signed-off-by: Yan, Zheng <zyan@redhat.com>
Merge pull request #3121 from ceph/wip-10277

Reviewed-by: Greg Farnum <gfarnum@redhat.com>