LU-18961 mdt: add safety checks in MDT_BOOL_RW_ATTR

LU-18961 mdt: add safety checks in MDT_BOOL_RW_ATTR

MDT_BOOL_RW_ATTR macro creates a store and show methods
for a boolean sysfs entry, but in process they use result
of mdt_dev() call without checking, but internally
it uses container_of_safe that could return NULL or
ERR_PTR, so we need to check the return value.

While we can probably pass in the decoded PTR_ERR,
since NULL is also an option, simplier to just return
-ENOENT which makes sense on it's own.

Change-Id: Ibbc62cf74dc5d8b7e335fc42ff5149bd2ae174c6
Signed-off-by: Oleg Drokin <green@whamcloud.com>
Test-Parameters: trivial
Fixes: d71a24005b753 ("LU-17144 mdt: set dmv by setxattr")
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/59009
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Timothy Day <timday@amazon.com>