fs/ntfs3: Add ioctl operation for directories (FITRIM)

While ntfs3 supports discards, FITRIM ioctl() command has defined
only for regular files. This may confuse users trying to invoke
`fstrim` utility with the directory argument (for example, call
`fstrim <mountpoint>` which is the common practice). In this case,
ioctl() returns -ENOTTY without any error messages in kernel ring
buffer, this may be easily interpreted as no support for discards
in ntfs3 driver.

Currently only FITRIM command implemented in ntfs_ioctl() and
passed inode used only for dereferencing NTFS superblock, so
no need for separate ioctl() handler for directories, just add
existing ntfs_ioctl() handler to ntfs_dir_operations.

Signed-off-by: Nekun <nekokun@firemail.cc>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>